i was thinking its probably not ur muffler dude, more likely ur cat, happens when they get old, turn into a smaller piece and rattle around in the housing there just slightly to big to fit into the exhaust, seen 3 or 4 cars that have done it in the last year. My solution when mine did it was put the gas on it and it blew the remaining rattly part of the cat right out of there like a shotgun!!