This has been on my mind for quite some time. It's getting worse and has become a Saga. I have a thump in the rear of my car. It feels like some one kicking the rear floorpan. It is both an audible clunk and a palpable thump.

Vehicle:
-01 AWM 1.8TQM, Upsolute Chip
-Purchased, 73K mi
-Currently, 79K mi

Conditions:
-Taking off from a stop sign, I'll often get two thumps when I fully release the clutch
-Every upshift, when I fully release the clutch, it will thump
-Quick heel-and-toe down shifts produce a loud thump
-Quick throttle-on at 2500 rpm in any gear produces a thump

I visually and physically inspected the motor and transmission mounts. I found the rear diff mount was sloppy (as all B5 rear diff mounts are), but not torn. I purchased Stern Diff Power Ring and Rear Diff mount Poly bushings. After the install, the car shifted smoothly and delivered power better, but the clunk is now more apparent than ever and the sound is more pin-pointed to the center, rear of the car.

Next, I put the car up and looked for anything banging... the Borla, the Stern Diff Power Ring, and loose bushings. I found nothing noteworthy, no signs of rubbing metal or looseness. I checked the center drive shaft carrier bearing for play or damage and all the axle flanges for looseness.

I found my rear diff fluid a bit low, but I've never heard low fluid cause any other noise than gear whine unless the gears have already been damaged. I plan on changing the diff oil, mainly to make sure there isn't a ton of metal chunks.

What I did find was some questionable play in my rear axles. Spinning the rear wheels quickly with the car out of gear will produce a thump in the rear diff, but I'm not sure if it's happening at the CV joints or in the diff itself.

My next plan of action is to change the rear diff fluid and remove the axles at the flanges so I can rule out the rear diff or the axle CVs.


Any thoughts? Comments? Experience? The last time I had a thump like this, I was working on a 996 GT3 with bad thrust washers in the limited slip rear. Seeing as this is an open rear diff, I'm simply worried I have excess backlash or even a bad bearing. Short of tearing it down, however, I'm not sure how to rule this out!

Thanks for your time and suggestions.


P.S. Recommend gear oil is 75w-90 for the rear diff, correct?

 

Sounds like there is play in the prop shaft. Could be the support bearing or a cv joint. I'm not an expert, and I really have no quattro experience, but I've been reading up A LOT lately, and came accross those posibilities as for a thump noise in the conditions you describe.

 

Rear wheel bearings?

 

CVs are definitely on the list of possible suspects. I need to unbolt them from the rear diff flanges to see if the play is in the diff itself or the CVs.

As for the carrier bearing, I visually inspected it and wiggled the center drive shaft around. The rubber isn't torn and I don't have the tell-tale vibration that usually accompanies a bad carrier bearing. It is designed to move back and forth to a point and I don't see signs of it moving excessively. The U-Joint looks ok as well.


Well, I have no play at the wheel itself. Additionally, wheel bearing noise is usually a road-speed growl or whine. Either way, if I'm replacing axles, you can bet I'll probably be doing wheel bearings as well.


Thanks for the suggestions, guys. This is a new one on me, so outside ideas are good to help me pinpoint the issue.

 

Is your spare tire secured properly? Trying to think outside the box here.

 

No, that's actually a good suggestion. Believe it or not, I checked it. My last car was an 02 Jag X; if the spare wasn't secure in that car, it sounded like the whole trunk was going to fall off the back of the car!

It's definitely a power delivery issue. Drivetrain, subframe, arms... I missing something I just can't seem to figure out. This weekend, I'm going after the rear diff and CVs to see if I can pinpoint the issue.

 

You sure its not the U joint? You dropped the exhaust heat shield to check that thoroughly?

 

My first thought after reading what you posted was the rear diff or driveshaft. Really hard to diagnose online without actually hearing it. We're not mechanics in real life, we just play one on the internet

 

I pulled the heat shield (PITA) and checked both the shift linkage and the U-joint with a prybar and by hand. I don't see any looseness or signs of rubbing, no etreme back-and-forth movement. It doesn't look like it's going to happen this weekend, but I'd like to pull the center drive shaft and the rear CVs and play with the diff and check it for excess backlash. Then of course, I'll carefully re-inspect the CVs, center drive shaft, and u-joint.

Unfortunately, I have to drive to work!

Hey, some of us are real techs! I'm "retired" for the time being, but I understand how hard it is to diagnose online. It's just nice to get some counter points. I wish I had a mini racing cam to put under my car and drive it to watch for movement...


Thanks guys. Unfortunately, my weekend doesn't look like I'll have much time for my car, but I'll keep everyone posted.

 

Update:

Clunk is now violent and happens whenever the car shifts weight! I've also noted a high-pitched whine that has become significantly louder at 40 MPH, pointing a big finger at a dying carrier bearing. I performed a total axle backlash test with the companion flange locked down to eliminate the possibility of a bad rear axle or diff bearing. I recorded about 20 mm of total backlash at the rear tires, well within the industry standard of about 25 mm (1"). I moved forward and found a clunking noise at the front of the rear diff. Excess play was noted at the rear joint of the center driveshaft when spun quickly.

Now, to order parts.

I'm thinking Blau-parts' center driveshaft repair kit. Any thoughts?