I have a sound like a bad bearing in the rear end of my 260z. The sound is not consistant. It gets worse at certain speeds. I just had the differential changed, because the mechanic said that was the problem (not)! The sound is still there. He said the u joints on the half shafts, and the wheel bearings seemed ok. Any ideas?

U Joints will clunk.

Wheel bearings sound like a rrrrrrr and will change pitch with speed.

My bet is wheel bearings.

Be careful, the location of the noise can be very decieving! I had the same symptoms as you are describing about 7 months ago and before I did anything about it my transmission seized in a parking lot!