Could be just a loose heat shield in best case or a broken up catalytic converter core in worst case. Get under it and bump some stuff around on the exhaust system. I had a little Geo tracker that made a hell of a racket, just as you say. Heat shield had cracked out under the bolt heads and it was rattling around. Friend just had to replace the cats on her Toyota truck because the cores had broken up and were rattling around inside. If it is the cat cores, the car may also lose power and get worse fuel mileage, as the exhaust is restricted.