00 v6 4motion humming from gas filler area

I've have a 2000 V6 4motion wagon and since 2000 we would get an intermittent humming sound from the gas tank filler area whenever it was 90+ degrees outside and we've been driving for a while. It doesn't matter if the tank is full, 1/2 or near empty. It's really noticable when inside our garage, but hard to hear outdoors. We found that opening the gas cap would temporarily resolve it. The tank would creek and settle and the noise would stop but we'd have to deal with the smell. Putting the cap back on too soon and the humming noise would resume I'm guessing when the pressure builds up again. We took it to the useless dealers and of course they couldn't find anything wrong with it even though we recorded the sound for them. This has been knawing at me for the last 9 years. Anyone know what this is?