After months of having a tailgate rattle on MK5 GTI I have eventually solved it!
I have emptied the boot, sat in the booth whilst being driven, fitted new struts, removed the plastic cover and changed all the metal clips, moved the boot bump stops..... it was the passenger side reverse light section! The light didn't move at all when pushed. it was one small rectangular plastic piece (i suppose something that should stop any rattle) at the top of the light (there are 3) that had become unstuck. Now stuck back on light bolted back on and all quiet!
Hope that helps someone