Firstly, big thanks for posting all the replies..
I was talking to someone else in an X5 at work, who heard it and said it was exactly what people have said, the tailgate. The dealer on their car put some blue stuff that left marks where it was rubbing so they could fix it.
Sounds like the stoppers / oil in the lock may be the way to go..
Re the tools... that had been one of the first things, and they are nicely wrapped up in a plastic wallet in the side and not moving it, but thanks for all the suggestions.
Luckily sounds like its fixable.
I was worrying it was the luggage rear separation net which I had raised a few times and had no idea how that would be fixed.
The only thing I need to remember now.. this car is somewhat higher than my last car.. as I found out when I opened the driver door against a huge flagstone kerb to hear crrruuunnch as the door scrapped across it.. wasn't a happy bunny, but so far I can't see any visible damage, and think its underside rather than cosmetic.. but I've not washed it yet which is when you see it
