Uh Oh. I just experienced several "clunks" from the rear end of my 97 M3. After the first one, my first reaction was that one of my kids had put a bowling ball in my trunk. The second made me head for home. This is not the kind of sound or feel from an unlubricated CV joint, but seems to be coming from the differential. It is a singular thump and shudder in the drive line that seems to occur under positive load and is proportional in severity to the amount of torque applied. It also seems to happen in both turns and straight line situations. It did not occurr, however, as I coasted for about .5 miles in a long gradual down hill section of the drive home. It did occurr as I gently drove up the following hill. When I reached my garage, I reversed into the driveway quickly and restarted quickly back into the garage in a straight line. This resulted in the most violent result of all. I raised the rear of the vehicle. All suspension and drive line components seem to be secure. When I spin each wheel alternately, it spins the oposite wheel in the oposite direction, but with no noticeable interference or noise. Oh yea, the fluid is tired, could this be the issue?.....What now?
For the biggest "Oh no", look at the body where the rear suspension subframe mounts. See if one of them has torn out of the body. If so, you're talking extensive work to make it right. Everything else is a walk in the park compared to that.
My clunking was a bad driveshaft U-joint. 1996 with +237,000 miles. Same exact symptom. Normal driving, no thunk. Hard acceleration, THUNK! First it was thought to a bad motor mount. YMMV.
I'm going for driveshaft as well, but it may just need a center support bearing. Either way it has to come out.
OK Thanks for the advice. All subframe mounting points are sound, so that's a relief. After things cooled down I was able to investigate further. When I apply pressure to the output flanges, not only do they move in and out rather freely, but up, down, back and forth (a lot) as well. I'm guessing so many different axis of rotation are not appropriate in this application. Also, I had the drive shaft off about six months ago and it looked to be in great shape, so I'm still leaning towards some kind of diff failure. Is this enough evidence to go this direction? Or should I still start with the drive shaft joint and support bearing?
I'm assuming it's a LSD. When my LSD starting going I could hear a whirring sound. Never had a thunk. Before I had my center load bearing replaced the first time, I heard a scraping sound, particularly in reverse. I had my driveshaft replaced along with a new guibo and center load bearing. YMMV.
