Noise eating my nerves

Hi guys, few days back, after 30 miles of high speed driving on a very damaged road I started to hear some slight knock coming from the rear, beneath the trunk. I emptied everything from the car and gave it a last try, there is definitely something loose there below..
It is noticable at low speed while going over bumps.. I removed diffuser, checked the heat shield, plastic protectors, exhaust tips and protectors..everything is tightened and this noise remains.
What could cause this noise? Bushings, shock absorpers,..?
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^^ this would be my first guess too. Just move it down (towards the front of the car) a little and it'll pull your boot shut a bit tighter. You might need to open/shut it a few times to check you've got a nice shut (not too tight, not too loose).
 
The rattle from the boot lock is a very common fault on the mk3's. we usually just pack the locking mechanism with grease, this stops it.
 
My wrench was a bit to big, couldn't turn the torx.
Instead I pulled this metal striker plate and this rattle is gone..for how long I can only guess..