Check your overflow hoses at the bottom of the tank. They may be pinched and it is forming a suction from the fuel pump drawing fuel out and is sucking in air to compensate for the fuel loss. If not that, unplug your fuel pump and see if that stops it. It may be staying on for some reason.
Mine does the same thing. The tank is sealed (for the most part) and gasoline is a very voilitial liquid... it vaporizez and the vapors expand at relativly low temps. The flash o[point is around 74 deg F. The whistling is the press from the engine and ambient heat causing the fuel to vaporize and expand.
Just, with 2 hands, pop the cap and reseal it. No worries. (anyone got a match?)