recent repairs have been rear hub replaced due to bearing issues and a differential was replaced as well - after reported noises coming from the rear of the car. am glad all was under warranty...

however, after the above repairs were done. now i hear a noise that seems to come from the rear. sounds like a light grinding noise or like a rock tumbler and strangely enough like bad radio reception. it seems to come and go and i am having difficulties replicating it. it seems to occur at moderate acceleration from stop and also from 35 to 40... left it with audi for almost a week - they heard the noise once, but when the tech got it back to the shop for a senior mechanic, they couldn't get the noise to repeat.

any thoughts and/or recommendations would be greatly appreciated.