Brake grinding @4-7 Mph

I have recently noticed a grinding noise coming from my front end. It happens intermittenly in stop and go traffic when I have to slow down it starts around 4-7 mph and happens for 1 or 2 seconds then stops. I do not hear it at 0-3 MPH & I only hear it when my feet is on the brake pedal and not if it coast down to that speed. Anyone have an idea as to what's going on here. Is it really the brakes or the tranny. My so called certified GM techs can't duplicate the problem when I turn the vehicle over to them. Any help is much appreciated