Over the past few weeks, my truck had developed a rattle when going over bumps that sounded like it was coming from somewhere in the firewall. While stopped at traffic lights, it would sometimes vibrate. It was driving me nuts! I was at the gas station yesterday and I got out of my truck and closed the door when I heard the same rattle. I re-opened and closed the door again, and there was the rattle again. Then it dawned on me - the roof rack! I put my hand on the roof rack and pulled back and forth on it, and sure enough, it's loose! The little screw with the allen key head on it that attaches the plastic corner piece to the tube needs to be tightened. Easy fix! So glad I finally figured it out!