3 door hatchback 1.2 Reference 2006 registration

Loud rattling noise from rear on bumpy roads - jack and wheel brace etc are all tightly secured, catches for folding rear seats have been oiled, panel under rear silencer seems not to be loose.

Suspicion now on the black plastic moulding fixed to tailgate as this is not tightly secured. Cannot figure how to remove this and check - any help much appreciated.