Undesirable Noise from rear end

[George Harris]

A fellow lister and I along with our respective wives went away for the
weekend to Limerock. Sorry we missed the other listers, but we did see
lots of Audis.

While sitting in the rear, a whine that started above 10 mph and was
definitely speed related, became a bit annoying. The owner thought that
the noise was due to his aggressive tire treads. The tire shop says it
isn't due to the tires, so we checked a few more things.

The oil in the rear diff was low, so we topped it up. Noise still there.

Checked all the CV boots, and there were no tears.

The hubs were still cool after a high speed run, so it isn't obvious
that it is the bearings.

With one rear wheel in air, there is a fair amount of play  and some
noise in the drivetrain when I rotate the rear wheel.

How would we determine what the problem is?

The patient is a '95 Model 90 Quattro with 140,000 Km on it.

Cheers
George