Wouldn't a loose heat shield rattle all the time and not just when the car is first started. This sound does go away after the car starts warming up. I am not sure if it is the cat either....
When the shield would rattle during cold, it was loose, but when it would heat up and get hot, it would bend and warp from the heat, and get tight in the mounts, and rattle would be gone. If you seen the chinzey holddowns for the heat shield, you'd understand how It could probably rattle
