Loosen the central bolt on the 2 rubber bump stops, one either side of the tailgate inside, then turn the rubber bump stop itself 1 or 2 turns anticlockwise - this raises them about 1mm, tightnen back up the central bolt and you're done. Mine went from rattling on every bump in the road to no noise at all with just 1 turn of the bump stop but I guess some might take more.

The central bolt is one of those star socket ones which are not in every socket set, but worth getting hold of one as getting more commonplace.

Hope this helps.