The rotors are slightly warped? I had a similar sound that was ultimately diagnaosed as warped rotors. Once the rotors were turned the noise ceased. Do you feel a pulsation in the brake pedal when braking?
The pads are near the end of their life? Check the thickness of the pads.
The calipers are hanging? Sometimes caliper pistons will not decompress which causes undue pressure of the pad on the rotor during normal driving. In this case the calipers still work fine to stop the car, but don't release fully.