Knocking from rear seat mounts

I have a problem with the rear seats knocking over bumps, if I grab the headrest and wobble the seat you can see it moving and making the knocking noise. This seems to mainly be around the bottom pivot joint.

I've wrapped some insulation tape around the drivers side pivot pin which has helped a little bit but the passenger side one isn't as easy to get to.

Has anyone else had this problem and been able to get it sorted?
 
I discovered the same problem the other day when I was tilting the back rest. A broken plastic bushing fell out from the seat so I guess that this bushing is the key.
The problem is that the dimension of the bushing is a little to small so even if the bushing is in place you still have a play.

I'm not an expert but that's my theory.

I'm new at the forum and from Sweden so please have that in considaration
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