Had similar issues with my '15. I'd like to think I had the curse of buying an early build and that these small problems have been fixed in future production, but maybe not.
I recommend you check the following when dealing with a rear rattle, especially at low speed.
-Make sure your jack/toolkit/chocks are secured tight.
-Make sure your rear cargo area floor storage bin is tight. Lift up the thin rubber liner and you'll find two hand dials to tighten.
-Check to make sure the upper portion of the interior trim, which surrounds the glass, is secure and looks to be where it should.
-The anti-pinch sensors can also be a culprit. Attached to both sides of the liftgate are thin strips of black plastic running along each edge. These are safety sensors actually, but I've found when they get cold, they harden and can be prone to rattles. I was able to get a very high-quality roll of adhesive backed felt, about the same width as scotch tape, and I was able to carefully place in between the pinch sensor and their substrate. The sensors still function, but it does insulate the plastic from each other. Again, you're just tucking this felt into the small gap on the sensor; it shouldn't be visible unless you've got the gate open and you're looking from the side.
It's unfortunate that we have these issues in trucks that are costing 60-75k, but hey at least I don't have buffeting and I'll admit I'm a perfectionist.
