A dry driveshaft center bearing can make an interesting noise -- a high-pitched whine with kind of a warble. I've had a couple Audi 4000 quattros, and they both needed bearing replacements when I owned them (the carriers were fine). The noise tends to sound like it's coming from the rear of the car and I never heard it when I was driving faster than about 35 mph.

Just a thought --

Russ M