I've had an unusal rattle coming from the tailgate area, especially with the onset of cold weather. Turns out that it's the rubber buffers that are pushed into the tailgate trim and contact the parcel shelf when the tailgate is closed. The buffers flex and move against the plastic trim, making a noise a little bit like 2 coconut shells being banged together, just like in the school play but quieter. Fixed by putting silicon grease between the buffers and trim.

interesting read i have something similar to this will have to try your fix!!