My muffler used to rattle around a lot too, I have a custom 2.25 Magnaflow. My buddy and I noticed that when I was driving around the muffler was bouncing around all over the place, and was really loosely coneccted. We just jacked the car up, and re-welded some of the hangers so that they would hold the exhaust tighter and higher. Also we realized that not all the hangers were being used, so we added some, and this seemed to fix the problem. No more rattle, and no more muffler breaking sounds over speed bumps lol. I dunno if this helps, but I mean you might want to check it out. Also, I don't have a heat sheild on my '99 lol, but I'd check to make sure yours isn't protecting the gas tank.