A lot of people don't say this when dealing with heat shields so I will...
When you get down, look around for the heat shields (I believe there's four or five) and knock on it. It should have a very high tin sound to it, not a low hollow metal sound. When you find it, it won't look loose. In fact, it'll/should still look perfectly intact. However, the reason it's making that noise is because the welding was poorly done underneath all first gen Xs.
Anyways, once you find the high tin noise, look to see if the welding has gone bad and sort of narrow out the two pieces so they won't be colliding too much anymore.
-Jonny