When empty it must resonate a lot. Some old 80s luxury car a flatmate had the tank had a 3-4cm thick foam between rubber pad on top of the tank to reduce noise. So guess there is something to it. But awd cars the tank is pretty solid since it has to clear the driveshaft.  

A friend put sound deadening on the fuel tank and said it helped a lot.. could be another place to think about.

Well I’ve not driven the car yet but just in the car park the reflected sounds have dropped noticeably while installing it. tapping of the treated vs untreated was obvious, not dead like the tar/rubber mat but no ring. Very surprised now much different given it’s so thin and light.   If the heat is reduced as well then will be wishing I did it years ago 😒

I don't think my WRX is as well put together as yours

Very little trim comes out and it’s all bolted near top so can’t move due to airbags etc so not likely to rattle. lights are also screwed in to roof so no new rattles for those either.   paranoid bit is bending the headliner when getting it out.