I've owned two Audi 4000 quattros. On both of them the driveshaft center bearing went dry and got noisy. It produced kind of a middle pitch, warbling sound -- kind of a wail vs. a growl or a shriek. It seemed to be worst at low speeds and would stop abruptly as I went faster.
I'd have someone drive your car while you sit in the back seat with a mechanic's stethescope or suitable substitute. Once the "music" starts you can trace along the driveshaft hump with your listening gear and confirm the source.