Brake noise


hi I have a 2004 bmw325i. The rear brakes make a loud noise like a semi truck brakes being applied at gentle or slow speed braking. high speed braking no problems. the noise is intermittent. plenty of brake pad left and the rotors are new. help please.

My car is doing the same thing with the front brakes, pads fine, rotor fine. If I clean the dust off and run sandpaper on the rotor surface and the pad surface the noise goes away for several days. A friend who works at Advance Auto parts thinks the rotor has a hot spot on it.