Drivetrain Rattle

[Daniel R]

I have been hunting down a persistent rattle for some time now. It generally happens when coming off the clutch and accelerating from a standstill. It is much more pronounced when starting uphill, the car actually sounds terminally ill for a few seconds.

To give some background, it always felt like a drivetrain issue and I assumed it was a clutch and/or flywheel on it's last legs, possibly with loose clutch bolts as this is what it sounded like to me. I had those replaced with a LWFW and sprung clutch, no change at all to the rattle, some new vibrations due to the single mass unit but I was prepared for that.

The next suspect was a pulley or tensioner. The water pump was only a few months old and the rattle predated its replacement so not that. I replaced the entire alternator as the clutch in the pulley was not working as it should and while we were in there... At the same time all the other pulleys and tensioners were inspected and everything was working as it should.

Exhaust rattle? Seriously doubt it since I have no cats at all (ROW setup only has one set of O2 sensors post cats so no CEL) and I have replaced the engine and transmission mounts with semi solids so I doubt that the engine is torquing enough to move much of the exhaust.

Now that we have most all of the red herrings out of the way, what could be causing this rattle? Some slack in the drivetrain somewhere is what it sounds like to me. Where should I start looking? Is there a big bolt labeled "Tighten Here To Stop Rattle" type solution or is this another difficult case?

All suggestions are much appreciated.

[Hayden]

Is this a rattle only after the clutch is engaged, or does the assembly sound and feel more like two wobbly plates banging into each other until some power, and revs straighten it all out? Could be input shaft or throwout bearing.

Describe the sound more.

[Daniel R]

Originally Posted by Hayden

Is this a rattle only after the clutch is engaged, or does the assembly sound and feel more like two wobbly plates banging into each other until some power, and revs straighten it all out? Could be input shaft or throwout bearing.

Describe the sound more.

The rattle is only when coming off the clutch and accelerating slowly and it is more pronounced when under a load (ie. uphill).

It is more like your former description of wobbly plates, a clack, clack, clack type of sound and once the clutch is all the way out and we are accelerating it disappears completely.

Also, the throwout bearing was replaced with the clutch a few months ago so that has been ruled out already.

Is that any further help?

Thanks for your reply, Hayden.

[Nine8Six]

I'd be very curious to know about methods used by the professional in the trade to detect the origin of strange rattles under cars. I understand this can't be done with the car lift up in the garage in most case. Unless of course it is a common rattle sound.

Have you had a Pcar expert driving your Boxster to reproduce the sound yet?

[Hayden]

Originally Posted by Daniel R

The rattle is only when coming off the clutch and accelerating slowly and it is more pronounced when under a load (ie. uphill).

It is more like your former description of wobbly plates, a clack, clack, clack type of sound and once the clutch is all the way out and we are accelerating it disappears completely.

Also, the throwout bearing was replaced with the clutch a few months ago so that has been ruled out already.

Is that any further help?

Thanks for your reply, Hayden.

When you replaced the clutch, was the pilot bearing also addressed?

The noise was there prior to the new clutch, correct?

When the car is warm, at idle, is there a significant change in noise when the clutch is depressed? The single mass flywheels might amplify this effect, so might not be as useful for diagnosis, especially since the throwout bearing was replaced.

Time to look at the input shaft/pilot bearing side in my opinion, but please keep in mind that I'm not as experienced or knowledgeable with this car as most on here. You said all suggestions appreciated Maybe this will bump the thread and get noticed by someone who can help more. Best of luck!

[Daniel R]

Originally Posted by Hayden

When you replaced the clutch, was the pilot bearing also addressed?

The noise was there prior to the new clutch, correct?

When the car is warm, at idle, is there a significant change in noise when the clutch is depressed? The single mass flywheels might amplify this effect, so might not be as useful for diagnosis, especially since the throwout bearing was replaced.

Time to look at the input shaft/pilot bearing side in my opinion, but please keep in mind that I'm not as experienced or knowledgeable with this car as most on here. You said all suggestions appreciated Maybe this will bump the thread and get noticed by someone who can help more. Best of luck!

Do you mean whilst stationary? Clutch in or out when in neutral makes no appreciable difference, maybe very slightly although I would put this down to the sprung clutch, it is barely perceivable.

The only thing you've mentioned that resonates is that this problem is much more noticeable when the car is warm.

Just a thought, if the inner CV joint/s were failing could this be a cause?

I am getting desperate to solve this problem as it affects the drivability enough to become a real nuisance and I have already spent quite a bit of money replacing "the usual suspects" only to find that nothing has changed.

Thanks again for your help, it is much appreciated.

[woodsman]

Is the clutch disc a high-friction/HD/long-lasting, sprung type? Mine is and after I've been in stop and go traffic for a bit, something starts to 'hop and bang' as I'm nearing full release. I've attributed this to the high- friction, long lasting, sprung disc and high force, pressure plate. It disappears if I purposely ride the clutch. It never does it when the car's cold. I've also observed that when it's occurring, I am failing to release the clutch. Why? Because the car will catapult forward in a neck-snapping fashion or it will stall. SO what I'm missing is a little slip while 'things' synchronize to the same speeds. I have a 'Grabby clutch' because it's designed for circumstances outside of the OEM design parameters. Hence the nasty behaviour. I WAS going to turbo my car. Same problem or not?

[Daniel R]

Originally Posted by woodsman

Is the clutch disc a high-friction/HD/long-lasting, sprung type? Mine is and after I've been in stop and go traffic for a bit, something starts to 'hop and bang' as I'm nearing full release. I've attributed this to the high- friction, long lasting, sprung disc and high force, pressure plate. It disappears if I purposely ride the clutch. It never does it when the car's cold. I've also observed that when it's occurring, I am failing to release the clutch. Why? Because the car will catapult forward in a neck-snapping fashion or it will stall. SO what I'm missing is a little slip while 'things' synchronize to the same speeds. I have a 'Grabby clutch' because it's designed for circumstances outside of the OEM design parameters. Hence the nasty behaviour. I WAS going to turbo my car. Same problem or not?

I'm tempted to say yes, as I too have what I suspect to be a very similar setup to you. The only difference is that this noise was EXACTLY the same with the OEM dual-mass flywheel and solid clutch setup. It is true that riding the clutch stops the noise, if I give it some gas up to 2k rpm then come off slowly the noise is gone or at least greatly subdued. My theory on this is that there is something loose somewhere which gives driveline play and a progressive application of torque keeps things at an extreme in the range of play. When the clutch is slipping at semi-engagement the torque is not progressive and it causes the component that has some play in it to bounce around between the maximum and minimum range of play, hence the noise as it bounces back and forth.

I hope that my explanation can be understood, I am not a mechanic or an engineer so please forgive my poor attempt.

The question really is what can be causing that much play in the drivetrain?

[evomind]

You really need to get the car up in the air and bang around on EVERYTHING.
At this point I would not rule out a loose heat shield, panel, or something. Also I had a muffler with a bad resonator inside, whatever its called INSIDE the muffler come loose and cause exactly what you are describing.
A thorough underside exam hitting parts is first step.
Also, is the vibration/noise you hear RPM related or speed related?

[woodsman]

I think the clutch disc is bouncing off the FW instead of pushing against it. This because the friction at contact is so great relative to the clamping force of the pressure plate. If I dump mine it sticks and there's no banging. But if I'm gradually releasing than it works okay for a bit but THEN starts banging (bouncing?). When slipping I think most clutches would be transmitting a constant rotational force but mine heats up and STICKS ( refusing to slip). Revving to 2-3000 rpm improves things on my car as well. My disc is a segmented, Kevlar reinforced one. It is supposed to last a lot longer even under high horsepower applications. I say it's too much of a race clutch for me. The next time it's apart I'll put in an organic sprung one if I can find one. On my DD I don't need to give throttle while releasing the clutch but I'm learning to do so when in the P-car. What type of disc have you?

[JAAY]

A bearing could have let go in the tranny. There are sealed bearings in there like the ims so I've been told. I am on my third tranny due to a rattle. My latest one I think I took a tooth off a gear. I haven't opened it up yet to inspect it.

[Daniel R]

Originally Posted by evomind

You really need to get the car up in the air and bang around on EVERYTHING.
At this point I would not rule out a loose heat shield, panel, or something. Also I had a muffler with a bad resonator inside, whatever its called INSIDE the muffler come loose and cause exactly what you are describing.
A thorough underside exam hitting parts is first step.
Also, is the vibration/noise you hear RPM related or speed related?

Welcome to the forum evomind!

It is a pleasure to find a first-time poster offering advice to someone else, rather than the usual questions of "Why is my Boxster now a smoking heap on the side of the road? Will it be expensive to fix?"

I suspect you are right, I really must diagnose this one properly. I've spent far too much time and money guessing and replacing with no positive results.

Hope to see you around the boards.

[Daniel R]

Originally Posted by woodsman

I think the clutch disc is bouncing off the FW instead of pushing against it. This because the friction at contact is so great relative to the clamping force of the pressure plate. If I dump mine it sticks and there's no banging. But if I'm gradually releasing than it works okay for a bit but THEN starts banging (bouncing?). When slipping I think most clutches would be transmitting a constant rotational force but mine heats up and STICKS ( refusing to slip). Revving to 2-3000 rpm improves things on my car as well. My disc is a segmented, Kevlar reinforced one. It is supposed to last a lot longer even under high horsepower applications. I say it's too much of a race clutch for me. The next time it's apart I'll put in an organic sprung one if I can find one. On my DD I don't need to give throttle while releasing the clutch but I'm learning to do so when in the P-car. What type of disc have you?

It would make for a very big coincidence if this were the case, as the noise was exactly the same with the old dual-mass and toasted clutch, also, the noise developed when the old clutch still had 20% left in it. Then again, coincidences do happen.

The clutch is a HD pressure plate and a high-friction, organic disc. I didn't go with the kevlar discs and I am glad. It is annoying enough in traffic with the setup I have and I rarely use it for what it is designed to do. I must admit that I like the LWFW although my tolerance for discomfort and NVH is probably greater than most.

[Daniel R]

Originally Posted by JAAY

A bearing could have let go in the tranny. There are sealed bearings in there like the ims so I've been told. I am on my third tranny due to a rattle. My latest one I think I took a tooth off a gear. I haven't opened it up yet to inspect it.

Yes, this is the type of thing I am dreading. Still, a $5k fix and my 2nd gear problem would be gone too... it is the only thing that prevents my wife from driving the car, the fact she misses 2nd gear 1 shift out of 5. It would save me a lot of stress the times when I need her to drive home after those long and lingering Friday afternoon lunches with the obligatory several bottles of red wine.

Every cloud...

[epapp]

I've got the same lurching, bumpidty scramble near the top of the clutch on some slow releases. My stock clutch is nearly gone (pedal engaging at the top) and I had just assumed the clutch was fried down to the rivets. I live in a hilly area and it certainly chops around after I've hit a few stop signs trying to accelerate again uphill. I can release quickly and won't get any jitters. I suppose if you guys have new clutches it isn't the clutch wear that's causing this. Hmm....


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[Daniel R]

Originally Posted by epapp

I've got the same lurching, bumpidty scramble near the top of the clutch on some slow releases. My stock clutch is nearly gone (pedal engaging at the top) and I had just assumed the clutch was fried down to the rivets. I live in a hilly area and it certainly chops around after I've hit a few stop signs trying to accelerate again uphill. I can release quickly and won't get any jitters. I suppose if you guys have new clutches it isn't the clutch wear that's causing this. Hmm....


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Yes, this seems to be the closest description to what I am experiencing. My old clutch engaged very near the top, just as the new one does. A couple of inches of engagement is all that is really needed. I thought that the old clutch was worn to the rivets also (it turns out it was ) and I assumed there were some loose bolts in there as that is what the sound most resembled.

I did come across something on another forum regarding inner cv joints. Is this possible? It would be a relatively inexpensive fix if it were the case.

[epapp]

I feel that it is related directly to engine rpm and clutch behavior (just from experiencing the symptom). If it was inner CV joints, wouldn't it happen every time a certain amount of torque was put on the axle (and NOT just when in first gear, slowly releasing the clutch?)?

[epapp]

Interestingly enough, I just remembered something I did that also was sort of related. I was engine braking down a short, but steep hill in first gear, and (riding totally on the engine) I pushed in the clutch and felt sort of the same jolt, just once though. I assumed it was because of the engine braking torquing the motor and when I pushed in the clutch, the engine sprung back to its natural position. The two could be related...maybe when motor mounts are old and you hit the clutch with just the right frequency, it shudders the engine into a bouncy wobble?

[Daniel R]

Originally Posted by epapp

Interestingly enough, I just remembered something I did that also was sort of related. I was engine braking down a short, but steep hill in first gear, and (riding totally on the engine) I pushed in the clutch and felt sort of the same jolt, just once though. I assumed it was because of the engine braking torquing the motor and when I pushed in the clutch, the engine sprung back to its natural position. The two could be related...maybe when motor mounts are old and you hit the clutch with just the right frequency, it shudders the engine into a bouncy wobble?

My motor and transmission mounts were replaced when I did the clutch. Replaced them with Wevo semi-solids so I can safely say it isn't the mounts, everything is tied down fairly tightly.

[epapp]

Today it also shuttered when it was cold, about 100 feet away from my parking space. I don't get it :troll: