There is a vent line that runs up into the fuel door cavity. It vents into the cavity on the front (forward most) wall of that cavity, about 1/2 way up or a little more. You can feel it with your finger if you reach in between the open gas door and the aforementioned front wall.
I fixed the noise by putting a rubber bottle stop in that vent, with a small ~1/8" hole drilled down the center so it would still act as a vent. But it no longer resonates and makes that very irritating noise.
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