I had the same issue. It started when the rear shelf was removed for the window tint, but sounds like it would have started away. The tinter was really helpful and took the shelf off again for me and we put some extra padding under the shelf and it helped a bit for a while.

As I didn't really fancy removing the shelf again (as it was a bit of a job) I got some foam from clark rubber and from the boot you will see a few large and small holes which are under the shelf. I placed a few large pieces of foam in the holes and after a bit of trail and error I have got the noise to stop. I would say it's 99% good now.

Hope that helps

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MY11 WRX + 86