For anyone who doesn't know, there is a TSB for the fuel tank rubbing against the chassis when its hot (90+ seems to be common) and the tank is full. This applies to all cars (years and models) for possible reasons as stated in the TSB:
1) Foam around tank moved due to adhesive failure
2) Foam was improperly installed during factory process, causing it to fold in places when the tank was put in
3) Foam was never installed at the factory
The TSB has a picture outlining 2 pieces of foam at the bottom, showing an example where one side is misplaced or folded. Instructions say you need a special tool to look behind the fuel tank tight gap.
A dealer master tech said they would need to drop the whole fuel tank in order to access the areas where the foam should be. They would then ensure that all foam is installed and use some heavy duty American glue to prevent it from moving. In order to do this, they'd have to disassemble the whole front of the car in a full day's work before they can remove the fuel tank. I was advised its best to try something simple like shim it up top, where we heard the sound after manhandling the tank back and forth and discovered that a hard brake downhill pushes the tank forward and gets rid of the sound (up until you go uphill or accelerate hard).
I really don't want a dealer disassembling the whole front of my car for this noise because it only happens when its really hot and a fuel tank. When I had my x73 suspension installed the other day, I requested the Indy shop tech/owner take a look under the car to see if he can find anything obvious. He took a look (got some pictures too, but I don't have them) and found that the bottom of the fuel tank sides toward the the cabin clearly had no foam installed and were touching the chassis (or within a few MM). He flexed the tank and squeezed in some foam panels between the chassis and tank (back side/ toward driver side of the tank), but no glue was used because the fit was tight making it hard (glue wouldn't allow pushing the foam in place).
I've driven plenty with 95+ degrees outside, a full tank of gas, and bounced the car around. I'm happy to report that there is no more squeaking or any other noise coming from behind floorboard.
Something I want to make clear though...there is a chance the foam that you're missing (or has moved) is in the center of the tank, which would not be accessible under the car. In that case, you would have to remove the whole tank, but its possible that if you shim the bottom of the tank (or add foam if its missing), this will push the tank away far enough from the chassis in the center that it will fix it because the tank would not be touching the chassis. My sound was clearly coming from behind the brake paddle, but if your sound is coming from behind the instrument cluster or higher, its likely the center foam is the cause (not the bottom).