ok....usually on hot days, every once and awhile, i hear a moan come from the back of my car, like, slightly high pitched. I noticed it once at the staging lanes at the drag strip when it was about 100 out, and i noticed it today when driving around. It starts, then stops. Ive never had it happen when it was cool out. Any ideas? the only thing I can think of back there is the fuel pump...but how would that make that noise?

 

Does this happen with the engine running or off? Could be pressure in the tank built up and then releasing through the vent tube and one way valve?