wickedwiggles

Alright, I have stop-tech slotted and drilled rotors with gold c max brake pads. I have a weird buzzing when I brake from the rear of the car and can't figure out what. Yes I would think it might be the rotors but it didn't make this noise before I replaced my wheel bearings. So its not the rotors, the dealers tried to tell me this too since it wasn't stock rotors it had to be them. Except the fact that all the wheels have drilled/slotted rotors and it only come from the rear, where less braking is done anyway.
So any ideas as to what it could be? It sounds like a high pitched metal grinder when I brake at high speeds and at low it is like a clicking, however only when I brake = (