I have a 2004 F-150 4x4 4.6l and I am having a sound coming from the rear when I have the back slider open. It sound like a loose heatshield or someting like that. its not all the time, but I have been under the truck banging on exhaust pipes, heatshields, cats, and I still can't find it. I've replaced the IWE Solinoid, no luck...I'm stumped! any ideas???