"Thwack" noise when taking foot off gas....

[Bluemill]

I have a 2000 Coupe with 85,500 miles. I just started to notice what sounds like a drive train noise when I take my foot off the throttle- suddenly, mostly when I'm in a lower gear, or almost a downshift/braking situation. It's a dull thwack or sharp thump, and it sounds like it's coming from the forward part of the car.

It is not as pronounced as when the splines in the rear axles get hung up that let go with quite a "THWAKKK"" when backing up in a parking lot, etc.- did the grease thing, and that fixed it.

How about a motor mount? Perhaps a loose u-joint in the drive shaft? I hate to guess this one- a problem with the rear axle?

Any ideas would be appreciated.

Best,

Bluemill

 

[killain]

'Thunk Noise'

OK, this is my opinion only, but "IF" you get the 'thunk' immediately upon lifting your foot off the gas pedal, I would have to do the simplest thing first, so I'd flush and change the 'Differential' fluid, if that fails, then I'd put the car in gear only and put it on a lift and rotated back and forth and see if you can replicate the sound or seeing id there is abnormal slop in the drive-train, and go all the way from the back axles all the way to the engine. Good luck !

 

[Hib Halverson]

You're on the right track thinking about engine mounts. Inspect all your powertrain mounts (front and rear) for both condition and tightness of the fasteners.

As for u-joints, a C5 has no universal joints. There are flexible rubber couplings at both ends of the propeller (drive) shaft. Hope that it's not those couplings which are making the noise as repair/replacement of the prop shaft require removal of the entire driveline and the cost of that is not going to make you happy. I'll add that if the noise is coming from the front of the car, I'm going to guess it's not a problem with the prop shaft as, if there was a noise from there, you'd hear it coming from the front or the back of the trans tunnel.

It is unlikely a noise from the front of the car is being caused by rear axle lubricant, however, if there is a "thunk" noise that originates from the rear axle, my guess is a lubricant change is not going to fix it. A "thunk" from inside the rear axle is likely a significant mechanical problem.

It will be difficult to reproduce the powertrain "thunk" with the car on a rack and the rear wheels hanging free because, if the noise depends on changes in torque, it probably will not repeat as, with no load on the powertrain, little torque can be generated and, thus, little torque reaction (what might be making the noise) is created.

Good luck.