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Why does an Audi hum...( besides not knowing the words )

OK guys,

My '86 5kQ started making a humming sound on the way to work Tuesday.
It continued all the way back home (35 miles) that night.

The humming sound is from the drivetrain, increases with speed, and does
not change under hard acceleration ( more torque through the drivetrain)
It would seem to indicate a wheel bearing, but there is no detectable
change when swerving from side to side. just a monotonous humming.

Yesterday evening I jacked up the two front wheels and ran them at low speeds
with the rear wheels blocked and with parking brake on. I was trying to
determine if the driveshaft middle bearing was bad. The noise seems to be
coming from the area of the gearbox ( 5 speed 016-AWD), but very hard
to tell if it was the same humming soound I had heard on the freeway.
It may have been just the typical gearbox noise with the differential

What is the best way to narrow down the problem to one component.
The car has 160k plus ( intermittant odometer, soon to be disassembled)

Alan Cordeiro