Well a long story short, a bunch of us from STL went to the Tail of the Dragon this weekend and had a blast. On our way to the hotel I let off the accellerator and heard a knocking sound from the rear of the car. It happens when I coast in neutral as well. The sound is coming from the differential after about 4 hours of working on the car on Saturday in a motorcycle shop (Cherokee Custom Choppers in Andrews, NC). The owner of that shop was incredible and didnt charge us anything for using his tools and garage space. Mike Pike is his name. Anyways, we pinpointed the noise to the rear end. Made the ~500 mile round trip back home with no issues and even raced a couple of the other M3's that went. The noise isnt present when I accellerate. What could it be? Mike, the owner of the bike shop thought it could either be the spider gears or the carrier bearing that is in the diff. Any thoughts? This is my DD and I drive 100 miles round trip to work every day so the sooner I can get this fixed, the better. If it is in fact the diff, I will replace it with a 3.38. Please help.
Is it a single knock when you let off the accelerator or is it a continuing knock? Does it happen constantly, as in every time you lift or is it random? I get a single knock occasionally when I let off suddenly or rarely at times when I shift. Is that the one you're having?
It is a continuous speed sensitive knocking. In other words the knocking is faster when im moving faster and slowerwhne I slow down... I cant hear it when I accellerate so I am thinking it has something to do with load. Any ideas?
I thought I had a problem with the rear end or half shafts when this type of knocking occurred last month with my car. I turned out to be the upper shock mounts. The metal bushing was pulling away from the rubber allowing the suspension to hop unrestricted for the first 1/2" of travel from norm ride height. To check for this, raise the car off of the wheels to let the suspension hang and look with a strong light at the bottom of the shock mount. (you may have to drop the plastic dust shield around the top of the shock to see better.) My car is a convertible so I was able to access and inspect the shock mount from the top. Good luck.
Well it was my diff. Apparently the 3.15 had a tendancy to have some of the bolts on the lsd unit back out and get caught in the ring gear. I replaced it with a 3.38!
