I noticed the rattling sound coming from the back as well, and it drove me notes. I initially thought it was coming from inside the lift gate. Turns out the majority of the rattle was coming from the license plate, and the license plate border surround that’s placed overtop of the license plate, both of which are mounted to the vehicle with the same screws. Additionally, I noticed they were only mounted via the top two screws, and that there were no predrilled holes for the bottom two screws. I sat back there, with the music cranked up, and that license plate was rattling around like crazy.
After securing it with the two bottom screws, and using rubber washers with the screws (also placed rubber washers on top two screws), I eliminated the vast majority of the rattle.
Another option you can do, in addition to securing the license plate better, is adding Dynomat to your vehicles doors and lift gate. It makes a WORLD of a difference! Blocks out a ton of outside road noise, etc., but more importantly insulates the doors and lift gate, giving you much crisper sound. Cost is reasonable depending on how much of the vehicle you want Dynomatted, but worth every penny!