The sound was driving me nuts...even if the music was turned up, just the fact that it exists annoyed me. I tried several methods including the felt pads on the rubber mounts. It worked but not for long so I convinced the CEO to drive one day as i literally sat in the trunk and was able to pinpoint the culprit (at least on my X). Turned out to be the latch. I've heard from several people that this is due to poor factory trunk compartment installation with some of the X5's...some even have their rear hatch mis-aligned.

I set out to find a thick felt pad that i could cut and cover both sides of the latch (upper)...i ended up using the small felt-pad dots. I cleaned the latch with rubbing alcohol and stuck 2 pads on each side. A little extra effort may be required when closing the hatch but it did the trick...been two weeks now and i haven't heard a sound

If anyone wants, i can take some pics but it won't be till this weekend as the truck is garaged during the week.