Could be a busted belt (in the tire), a "goose egg", or similar. Or, a large bolt or other foreign object stabbed into the tread.

You want to:

1) NOT drive it until you:
2) Take the wheel off and fully inspect the:
3) Tire, brake rotor, brake mechanicals, and the rest of the important stuff.

__________________
-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition