Meatball, mine would occasionally rattle too if I was listening to music loudly with a lot of bass (but primarily going over uneven bumps/driveways, etc).
I put some stick-on felt like in the link I posted above; no more squeaks/rattles over bumps, and I haven't heard anything from that area when playing bass heavy music either. I bought the felt with tape backing at Walmart and got two 5"x4" sheets for like $2.50. Might be worth it to try that before digging into the tailgate.