Had a problem like this one. I heard a metal sounding grinding noise especially at low speeds. I was sure it had to be the rotors or brakes. UPon replacing them, the noise still existed. Luckly a mechanic at dealership heard of similar problems that was caused by the cares TIRES, belive it or not. Somethign to do with the sidewall rubbing.
Anyway.....replacing the tires totally got rid of the sound. Give it a try.