Wait until your car is nice and cool, at least an hour since it was last driven, muffler repair on a hot muffler is no fun. Raise the car and safely secure it on jackstands. Now you can shimmy underneath to see what's going on.
About two feet behind the engine you'll see the exhaust pipe heading toward the back of the car. Right about where your seats sit, there will be a thin metal shield between the exhaust pipe and the floor of the car. Pull, push, poke and prod this every way you can imagine -- you're trying to make it rattle. If it's loose, you'll hear the same irritating sound you've been hearing.
If you can't make it rattle, you'll have to start the car. I highly recommend that you do not crawl underneath your car while the engine is running. Instead, poke your head down there and listen closely to see where the rattle is coming from.