its supposed to move some, thats why there are the rubber bands that it hangs from, sometimes they tear or break and stuff moves a lot more than you would like.

use the palm of your hand and hit it,(muffler) see if it makes noise when bounced around.

i had a the worst rattle on my last truck, and the bumper cover was the cause, i went nuts trying to find this rattle, found it while washing the truck.