I had the same problem on the smaller side of the backrest, and I did solved it 10 days ago.

Look inside the pivoting-latch mechanism, there is a rectangular black piece of rubber.

In the closed position, the U-shape latch (fixed to body of car) rests on that rubber piece.

You'll find the rubber piece relaxed (height reduced), and therefore the pivoting-latch rubs with the U-shape latch, and causes this rattle.

All you need to do is to regain the height of the rubber piece to restrict the movement.

I used one-face-sticking industrial sponge tape, and my problem is solved.

If this hadn't solve my problem, then I'd have glued another 0.5-cm-height rubber piece into the original one.

Tell us if this solved your problem. Good luck.