That’s been a fairly common source of rattles in the mk6 and mk7/mk7.5 VW Golf.
A quick and easy fix is to wrap some electrical insulating tape around the metal loop that the seat back latches onto when it’s in the upright position. It removes any slack between the latch in the backrest and the loop, and should stop the rattling. .
I did this fix in my mk6 Golf shortly after I got the car and it fixed the rattling, which never reoccurred in nearly five years of ownership. If you frequently
lower and raise the rear seatback and the tape wears through, it’s quick, cheap and easy to replace it with some fresh tape.