I may be having the same noise. I thought it was my bed or the tailgate. I was closing the gas tank door & it made the rattle. It sounded like it was coming from the front of the bed next to the cab, so i took the palm of my hand & kinda "tapped" the front part of the bed & sure nuff there was the sound. Im gonna take it to my dealer & get them to look & see if some bolts are loose or something. You may try yours & see if its the same noise. Just a suggestion.