I have this, it might not be the seat belt though. Try unlocking the rear seats and banging them shut. Failing that, i fixed mine my folding a cloth several times over to make a pad, then trapping it between the seat and the trim near to the latch to unlock the seats. It's only a small amount of pressure but it works.
If it is the seatbelt, try pulling it all out then retracting it again. If it still rattles, take it back
