It might possibly be the plastic trim panels. I had to take all the plastic off and use black felt strips and glue them where the plastic met each other, and where it contacted places on the rear door. The main culprit is the heavy extra spare wheel that places alot of load on the door and causes it to vibrate. Your best bet is to get someone to drive the truck, while you sit in the back and and isolate the noise by pressing against the plastic pieces until you find a general area where its coming from. Its common on all the G wagens.