i reckon on 2 possibles
1)all modern cars don't use warning light for brake pads some use a kind of bent piece of tin that vibrates/chatters on the disc face this shows when the pads are worn low and need replacing ,you will also find this piece of tin is only fitted to one wheel not both (less cost per vehicle)
remove road wheel on scrapping side check how much material is left on the pads and check option 2 at the same time.
2) check the dust disc guard has not snapped around a mounting bolt hole allowing the guard to vibrate/chatter
wirecutterman
June 2009