Need some help

First time since the test drive in December last year that anyone has sat in the back of the beast.....

Well yesterday I noticed a knocking sound on the rear left hand side behind the back seat everytime I went over a bump, definitely inside and only happens when someone sitting in the back....

The sound is almost like the seat belt unit or rear set back is not either tight enough or the seat back is not locked correctly which it is

Any ideas or has anybody had this before or heard of it....

Thanks in advance