I really can't see what can be the cause of this rattle apart the cat, at least with the indication you gave. If the noise occurs only when the engine is running, vehicle stopped or not, and that nothing seems wrong when you move or hit the exhaust system, it couldn't be a broken exhaust braket. On the other hand, a worn cat doesn't always rattle when you hit it with something, sometimes this is only an internal part which is broken in the cat and only the gases or vibrations can make it rattling.

If the noise would occurs only when the véhicle is moving (and not vehicle stopped, engine running), I'd say that there are many other possible causes (transmission, loosen part somewhere, brakes, etc) however it seems that this is not the case.