I got this rattle when I go over any little bump, and it is constant, 2 mph all the way to 130 mph. I finally got tired of it and started to look for it. It turned out to be the mounting bracket that is closest to the rear bumper holding the pasenger side muffler on. the thin metal that the bracket bolts on to is cracked and about to fall apart, it has 2 spacers between the frame and the bracket that prevents a bolt from going in the top hole, so only 1 bolt is in it. They used the spacers so both mufflers tips would hang even. I will see if we can weld the crack up while I install my LT's, hope that will work.