Strange noises

Please can someone give me some advice.
We have a 2.6 SE A4, 1996 (right hand drive). After driving for about 20 minutes it develops a humming/whining sound. To the driver it
sounds as if it's coming from the left, and to the passenger as if it's coming from the right, so therefore we think it's in the middle. It's very difficult to pin down, but it sounds as if it's coming from under the handbrake, or maybe a bit further back. It doesn't appear to happen when the clutch is depressed, and is most noticeable as you start moving from a standstill, although it is constant on the move.
It's going in for a check with the AA on Thursday, but I'm dreading a large bill for the repairs, especially as we're trying to sell the car prior to moving to Australia.
Any suggestions as to what it could be would be gratefully accepted.
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Might be a bad center driveshaft bearing or a transmission mount or exhause hitting somewhere. On a liftthe Mechs should be able to find it.
Ashley wrote:

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Poss a seized rear brake caliper, piston seized & putting preasure on the pad & contacting the disk, drive it for several mins & the pad & disk heat up with the friction & get noisey, had exactly the same problem with my 96 2.6 A4,
Steve,

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