Very common in the one piece tailgates. There are existing threads on making sure wiring is anchored, license plate is tight and adding dynamat the inside of the hatch if all else fails. Of course this is all assuming your tail gate is properly adjusted and things like basic screws are properly tightened.
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.