weirdest thing... only about once a month, at low speed only, i get a metal on metal sound, and my pedal drops a bit, pulsates, grinds. i let off, get back on and the grind/pulse is gone. only happens about 1-2 times per month, and never at speed, only at about 10 to 15 miles per hour. part of me thinks abs issue, because when it grinds the pedal drops and does that pulse thing, but it is not a situation where the abs should be activiated (heavy braking at speed... my brakes have been inspected and they say nothing is wrong with them. any idears??