This might help...
I had a very bad rattle noise coming from the rear of my X-6.
It appeared to me that the trailer hitch was loose and was about to fall off the vehicle. (It was that loud) The noise would come and go which would really baffle me. It turns out it was the rear hatch. Sometimes after opening it, it would not rattle and other times it would. They specifically mentioned the arms that run the hatch up and down. I did note that the warranty repair order did not have any parts on it, so they did not replace anything. Try driving around with the hatch open and see if it goes away, or have someone else drive the car and lay back there and put pressure on the hatch and see if it goes away. Best, Brian