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Old 03-30-2011, 08:11 PM   #1
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Knocking sound coming from left (drive side) rear area

Hi all.

I'm getting a light knocking sound from my left driver side rear area when I make a right turn at very low speeds. I'm suspecting it maybe the inner CV joints but no grease puddles by the area. It's a 1998 2.5. There is no sound when going straight or turning left. No sound when going over speed bumps. Any comments? Thank you.

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Old 03-30-2011, 08:29 PM   #2
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probably not the same but, i had an issue with a noise that turned out to be the e-brake line hitting a wheel weight as the tire went around. it started off as a very light noise that i could only hear when i would apply the brakes, then grew to a louder constant noise as can be heard in the video...

http://www.youtube.com/watch?v=14iWPvomj9g&sns=em
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Old 03-30-2011, 08:43 PM   #3
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Start with the usual suspects: wheel bearing, CV joints, engine mounts.
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Old 03-30-2011, 09:29 PM   #4
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Might be a dumb question, but is the car still safe to drive? I'm about 120 km away from the Porsche indy where I go for all work done on the car.
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Old 03-31-2011, 07:27 AM   #5
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So I had a chance to slide under my car today, and from what I can access with no lift, I have ruled out the CV joints. both on the left look ok, no grease or tear marks, the inner on the right side had a little grease but it appears to just been engine oil from the engine casing and not the CV joint. The engine casing from that area seems to be very prone to oil seepage, but as long as it's not from the center area then I'm not going to worry too much.

Funny thing is the sound is coming from the left side not the right. But after taking it out for a drive tonight, I've isolated certain situations where the either knocking or light banging sound from the rear left drivers side to:

At the red light, I shift in N (tiptronic) pull the hand brake up. When light turns green, release hand brake, step on foot brake, engage to D, let off brake, let the car roll and accelerate as one would in heavy traffic. At that moment I hear a peculiar sound (most i've heard it in a roll is twice) light knocking, like something is loose or two parts are hitting against each other. It also happens when I release my foot off the brake after engaging D and let the car roll forward before pressing the accelerator. Third incidence that it make the sound is when I'm in reverse and back up very slowly, with my steering wheel turning right.

I thought I have isolated the times the sound was audible so when I got back to my garage, I've re-enacted all the possible scenarios that caused the sound but couldn't hear a thing. So i crawl under the car to check the parts I can see without a lift and everything seems to be fine. Any ideas?

Last edited by Rui725; 03-31-2011 at 07:50 AM.
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Old 03-31-2011, 08:10 AM   #6
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Is your car on the original front engine mount? Do you find 2nd gear a bit hard to engage sometimes? If you were driving behind your car from a stop, and you watched the boxster's tailpipe when the person takes off, does the tailpipe dip alot? Do you have any slightly odd vibrations associated with engine speed?

Those are symptoms of a worn out front engine mount that can cause knocking sounds.

Next up is this: is the sound, when you hear it, getting slowly worse? Is it related to the rotational speed of the tires? If the sound happens faster when turning left quickly (i.e. at higher speed) and slower when you are moving more slowly, then it's almost certainly rotational in nature.

Rotating parts that cause knocking sounds are routinely related to the wheel bearings and CV joints. If you've ruled out CV, consider that you have a bad wheel bearing that only creates noise upon a specific load. Listen for a worn out bearing - if you can't hear it, consider jacking the car up (safely! with wheel chocks!) and seeing if the wheel has "play" in it from a loose bearing. It should not move back and forth on the rear - if it does, your wheel bearing may be loose.

No matter what, as a general rule of thumb, an occasional light knocking sound upon a certain load is going to be safe to drive on (there's always an exception)....as it gets worse, start to worry more, but also realize that it'll become easier to identify
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Old 03-31-2011, 08:55 AM   #7
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Being a tiptronic car, It's hard to tell whether the 2nd gear is rough to engage, but, as a general rule, the car chooses 2nd gear when coming off from a stopped position in normal every day driving. From a range of 1-5, 1 being the worse, 5 being the optimal, i would say there it's a 3.25 with a bit gear slippage when the computer engages the 2nd gear off a stop. There is a small lag that causes a very slight stutter in the car when engaging 2nd off from neutral.

I reckoned it could be the mount, but upon acceleration, there is no strange vibration. Instead, the car feels better at higher rpm's and speeds. I do not have a record of whether the engine mount has been changed or not, but most likely it is still the original engine mount. I do feel there's a lot of movement on the top engine compartment right behind my head when hitting pot holes and uneven roads though.

Is there a way to check for worn engine mount when up on a lift? From what I've read the bona fide sign of a need for a new engine mount is strong vibration at 4k rpm, which I have yet to experience. I'm going to have to check the wheel bearings when I have access to a lift and a 2nd hand to help me out. One thing I learned about owning a Porsche is preventative maintenance and reading goes a long way, and most of this I have to give credit to the helpful online community.

Last edited by Rui725; 03-31-2011 at 08:57 AM.
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Old 03-31-2011, 10:05 AM   #8
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Completely missed tht you have a tiptronic - disregard my comment on shifting quality.

Here's the thing - a boxer six is inherently balanced but will still be rougher at lower speeds. Even with a broken engine mount, you should experience less vibration at medium high engine speeds - what you might get is harmonic resonance at certain speeds.

Anyway, I can't comment on the stuff behind your head, except to say that it's easy to access those things and check to see if they are on tight. IIRC, they are all tightened in back there by hand/latch. So if something is moving around, it probably became unlatched somehow. None of that shelf should move if it's latched properly, at least as it is on my car...

You can not truly inspect the engine mount until it's out of the car. Even from under the car, its true condition is hid behind some rubber buffers.

If your car's is the original, I'm betting 95% its shot. That being said, just because it's shot doesn't mean it's the cause of your clunk.

Other symptoms of a bad engine mount = feeling the engine move behind you (front to back & some twisting), watching tha tailpipe move a good amoutn when the car takes off pretty quickly from a full stop, and various vibrations and resonances.

It's basically a rubber ball around a metal support that holds your engine supported and stable, and the rubber rips and tears. I just started a thread this morning about my own recent replacement of the mount - you can see it in there and how ripped my 99 boxster's was.
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Old 10-18-2011, 05:28 PM   #9
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I have the same thing going on and cannot figure it out. It seemed to happen right after I did some exhaust work, so I am thinking when the engine torques, it may hit a pipe? Did yo do any exhaust work recently?

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