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.