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Old 02-15-2019, 08:15 AM   #1
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Clunk under seat on acceleration and deceleration

So, after a fruitless search, I present this...

My '01 with Tip has started emitting a single, distinct clunk or thump from under the general area of the passenger seat on acceleration or deceleration.

Recent work:
  • All four struts.
  • Front lower control arms.
  • Front downlinks.
  • Front ball joints.
  • Front brake discs.
  • Coils, plugs, and tubes.
  • Alignment.

I did all the work, and my first thought was a loose control arm. But the car was aligned by the shop my indy uses, so I assume if there was something loose in the suspension they would have found it.

I found a post in a 996 forum that suggested motor mounts, but that doesn't seem like it would generate a noise under the passenger seat. I'm not noticing any other issues related to this, but it's clearly not supposed to happen.

Any input appreciated.

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Old 02-15-2019, 09:13 AM   #2
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this might sound silly ... but sometimes i have a drink or metal coffee mug that finds it's way under the seat. it's not carpeted, it's exposed metal under the seat. and it definitely makes a noise if i accelerate or stop quickly. just a thought.
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Old 02-15-2019, 09:37 AM   #3
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Ft engine mount?
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Old 02-15-2019, 10:21 AM   #4
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It sounds to me like a rear trailing arm. The symptoms you describe are classic "trailing arm bushings worn out" noises.

Those are relatively easy to replace, and you probably won't need an alignment, either.
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Old 02-15-2019, 12:08 PM   #5
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Quote:
Originally Posted by rathayes View Post
this might sound silly ... but sometimes i have a drink or metal coffee mug that finds it's way under the seat. it's not carpeted, it's exposed metal under the seat. and it definitely makes a noise if i accelerate or stop quickly. just a thought.
It was a golf ball for me! Worn trailing arm noise most noticeable while coasting (little to no engine load) at low speeds while driving over minor road imperfections.
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Old 02-16-2019, 05:11 AM   #6
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I would put my money on the front motor mount. Had the same sound in mine, but it was 11 years ago.
Have someone watch the exhaust tips at the rear of the car when you accelerate and decelerate. They will rise and fall as the slop in the front mount allows the whole motor to rotate up and down. The culprit is the rubber bushing in the mount that deteriorates and allows the two pieces of the mount to slop around. Very common problem with the 986.
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Old 02-16-2019, 07:42 AM   #7
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+1 on front engine mount
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Old 02-16-2019, 07:47 PM   #8
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yep engine mount. I actually bought one of those function first poly inserts but when I removed mine the whole thing fell apart. Had to buy a whole new mount.
Put it in D thump, put it in R thump
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Old 02-17-2019, 07:25 AM   #9
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Quote:
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+1 on front engine mount
+1 on the +1
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Old 02-18-2019, 06:52 PM   #10
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Okay, front engine mount seems to be the consensus. I'll look into it. Thanks.

Further updates as events warrant.
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Old 02-20-2019, 07:28 AM   #11
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So, I was driving the car in to work this morning and while rolling along at 30, I put the car into neutral and mashed on the accelerator and HOLY CRAP SOMEONE JUST HIT THE BOTTOM OF THE CAR WITH A SLEDGE HAMMER!

So, yeah, I'm in agreement: Motor mounts. I'll add that to the list (AOS, front struts, tail light bulb holders).

This car is one project after another.

Which is actually a lot of the fun of owning it.
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Old 02-20-2019, 09:20 AM   #12
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Can these motor mounts be rebuilt using urethane? I just happen to have some urethane laying around after having rebuilt my 924S transmission mounts.
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Old 02-20-2019, 12:47 PM   #13
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Maybe? But a new center is only $20.
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Old 02-20-2019, 06:19 PM   #14
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Be careful if changing the material in the mount! A much stiffer material will likely transmit a LOT more vibration and noise to the passenger compartment.

I've read of people that changed to stiffer mounts, and hated the car after that.
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Old 02-21-2019, 06:33 AM   #15
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If the water pump has not been changed, its one of those "While you are in there" things.
It also makes it easier to change the motor mount.

You may want to put in a low temp thermostat as well. You don't need to replace the thermostat housing.

A good place to buy the WP and thermostat is (ware house 33 auto dot com) (remove spaces) Put RENNLIST in the discount code place and you get an additional 10% off
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Old 02-21-2019, 08:45 AM   #16
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Quote:
Originally Posted by JayG View Post
If the water pump has not been changed, its one of those "While you are in there" things.
It also makes it easier to change the motor mount.

You may want to put in a low temp thermostat as well. You don't need to replace the thermostat housing.

A good place to buy the WP and thermostat is (ware house 33 auto dot com) (remove spaces) Put RENNLIST in the discount code place and you get an additional 10% off
+1

I changed my water pump when replacing the motor mount. It made reaching those WP bolts at ~2 and 4 o'clock ridiculously gravy. Definately a "while you're in there".
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Old 02-28-2019, 05:20 AM   #17
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Okay, good call on the water pump. Thanks.

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