A4 quattro rear end noise

[Ben]

There was a lot of noise coming from the rear of my Avant before doing a
complete brake job (vented rotors, ceramic pads, etc), as expected. What I
did not expect is that much of the noise remains. What I thought was noise
from frozen calipers, was also a distinct howl from something else. Is there
an easy way to differentiate wheel bearing noise from differential noise on
the quattro rear end? Suggestions on diagnosis?

Peace,
Ben