[Hippo]

Checkout the compartment under the loadspace, the one that opens with car's key. Maybe its not closed properly.
Also the rear loadspace roller blind cover, can cause some vibrations on the tail side.

 

[Hippo]

Im sure its locked cause there isn't other way to get your key back, just check if something prevents thιs postern to seal tightly and knocking when offroad.
Happened on me.
Of course maybe its the back door or the glass of it, but leave it for the end.