Might be the header vibrating against the chassis. If you look under it, you will see the header may be contacting the top of the chassis plate. A design flaw courtesy of HPI. Mine rubbed so much it almost wore a hole in the header. I wrapped a piece of a silicone exhaust coupler around mine to stop it. Start it back up and get it to vibrate, then start feeling around the truck for it. It's more than likely not engine related at all.