Howdy -- every time I go over a bump I get a significant vibrating thumping noise coming from what seems to be the middle bottom of my 2003. I should note that the sound is more of a low freq thud as opposed to a high pitched rattle. In giving the muffler a good push and pull that doesn't seem to be this cause. Has anyone out there had this issue and fixed it by replacing, removing or tieing down the cat conv heat shield? Many thanks.