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