weird noise, dealer no help

[amremt]

Thanks in advance. A month ago i purchased a certified 05 s4 mt from a dealer and for the last week it has been making a dull noise that sounds like someone is sharpening a knife on the back rotors. The noise is most noticable between 15-20 mph and cannot be heard when going faster. I took the car back to the dealer twice and both times that they looked at it they said that the pads and calipers were fine and there was nothing wrong. They told me that often there is break dust buildup and this was probably what i was hearing. Immediately after leaving both times the noise was still present. Has anyone else heard of this? having never owned an Audi before and since the noise wasn't there when i bought it, this doesn't seem normal. Any advice or recommendations are greatly appreciated. Other than this i cannot get over what a fun car the s4 is to drive and hope i don't regret going over to Audi's.

[RAudi Driver]

redo your own rear brakes and be done with it. It's won't cost that much and you won't have to fight with the dealer. I am so glad that my car didn't come with Audi service. My last one had it and I hated taking the car to them but they told me that if i didn't, they could void the warranty for no service.