I had this exact issue from the exact same spot you're claiming and with the exact same model car.

I determined it was the rear parcel from that first minute on a cold start where subarus idle real high, so let it idle and felt around until i stopped the noise.. yep was parcel shelf, right in the middle.

As someone said theres a whole guide on NASIOC to stop it.. but I can't be bothered pulling whole parcel shelf out and dynamatting it etc.... so i just cut a square out of a stubby holder and jammed a piece of it between window and parcel shelf.. all fixed.

Doesn't look very professional but not a single person has noticed it there. I may experiment with other ideas like getting some small chair feet (the little circle foam pads) and sticking them under it, so it's even more subtle.. but meh this does the job for now.

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MY11 WRX STi [SITDOWN]