Aaahhhh... the never ending boot rattle!
Rattle when your going over the painted seed bump lines could be...
I had this and found that it was the 6 screws that held the boot lock in place where loose. Noticed it the other week when I open the boot the black plastic surround moved.
Sorted that rattle, but know a new one has appeared. Gonna leave it be, well until I get pissed off again!