I understand not wanting to get the dealer involved, but I'm not handy with issues like this. Truck is brand new and has less than 150 miles. The dealer is correcting issues with the 3M film and a bumper issue next week. I figured it would be good to go ahead and get the rattling issue on the record with GM. I don't go to the dealer for service, just recalls and warranty issues. If you find a fix, please keep us informed.