DMTF-22

I am having the problem of a rattling noise from the undercarriage when in Drive or in Reverse. When I am stopped in my truck after a small amount of driving my truck makes a loud raspy kind of rattling noise as if a rock is stuck in between the catalytic converter and the heat shield? Had the problem checked out by a Ford dealership and that's what they said but I replaced the clamps after removing the heat shields and got nothing...It's worse when I have the AC running as well. The truck doesn't run any different but I would like to fix the problem if there is one and stop the annoying noise