Jimm 12/16/2011Several possibilities; worn / loose front or rear wheel bearing, worn brake pads, worn / warped brake rotor - or trapped stone - as you have suggested. Pull the wheel to check the brake condition - maybe consider having it checked by a competent and reputable repair shop.
Any abnormal noise heard is an indication that a condition needs to be checked. Try the 'How-To-Guides' feature on this website for specific steps to check and diagnose the brake system.
don 12/16/2011if you have drum brakes it could be a spring loose in the drum .....