I agree with Paragon. It is probably a heat shield. I remember this happend to me one time in my BMW, and some how a damn rock got on top of the shield and made a buzz at like 3400 RPM...I found it after about a week.
Also on a Jetta (at about 200 Miles on the car) I had a rock or something get into the vent on the front disc brake...so at slow speeds it sounded like a small bell sound...I took the rotor off and could not get it out for the life of me. So I bought a new disc.
