Hi Joe...welcome to the Forum
I would probably try another dealer to see if they can resolve or even diagnose the fault...seems unacceptable for a vehicle this age.
In the meantime, it may be worth seeing if there is anything in the boot area that is rolling around, otherwise it may be a bolt or assembly that has been dropped into a void during assembly and has been left there....seen it lots of times in cheaper and older vehicles.
Let us know how you get on with it
Cheers, Trevor