1988 5ksq "Humming" noise

[Saulius Mykolas <Empfader@stratos.net>]

I recently started to notice a humming noise( while driving ) coming
from what seems to be the rear of the car. My first thought was that it
is being caused by the tires. I have heard that this model had problems
with the rear disk brakes. Could this be a potential cause? What about
the quattro drivetrain or wheel bearings.  The sound is present and the
same when in neutral or in gear. Any comments/suggestions would be
greatly apreciated.
Saul