Noise from the rear. 2001 A4 Quattro Turbo 4CY AT

Hi guys and gals,
My A4 has a strange noise coming from the back. First it sounded like a humming vibration. In the last two days another noise had appeared which
sounds like buzzing hum.
Both sounds seems to change in pitch with the speed of the vehicle but not with the rpm (I put it in neutral for a few seconds while rolling at 40 rpm and the pitch didn't change). I have searched the group for possible problems but could not find any relevant posts. I have read about rotor warping and some brake problems and thinking they may be a cause. Has anyone have any ideas what the noises could be? Also what exactly is the rotor warping?
There are two Audi dealers where I leave but they both have bad reviews. We have a good local medium sized private garage. Should I take it to them? We also have a VW dealership which looks decent.
Thanks for any advice you can give me.
Dave
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Sound might be your fuel pump going out.
Doesn't sound like rotor problem but if you feel the wheels to see if they are hot after driving for a while that should tell you. They should be cool to the touch or at least not hot. If the brake rotors are dragging they create heat which will be transfered to the wheels.
Change to the independent (if they have Audi experience and recommended by other Audi owners) or VW dealer if you do not have warrantee left.
Tony '91 100q 5spd '98.5 1.8TQ 5spd
Dave wrote:

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Rear wheel bearings, guarenteed
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You don't say how many miles are on the car but I'd guess wheel bearings too. Since it appears to depend on car speed alone not engine RPM, that would seem to preclude something like the fuel pump. If it sounds the same when the brakes are lightly applied, that suggests the brakes are not involved. Still, that could be easily checked by any competent mechanic. Does it sound any different when cornering as opposed to going in a straight line? I doubt too that a CV joint is involved (that problem is usually more evident on low speed turns) but the CV boot integrity should also be checked.
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