Tank Assembly problems of the 2007 Toyota RAV4

I bought my 2007 Rav4 in November 2006 and took the car back to the dealer after a couple of months because of this humming/whistling/whining noise that others have complained. However, my humming noise is intermittent which makes it hard to find, and it seems to be more prevalent in the warmer months, not the colder months. I asked my mechanic about it, but he couldn't recreate the noise the day my car was in the shop. However, I intend to take it back there if I'm ever driving it in the area and the noise crops up again. To me it sounded like some kind of vapor pressure noise and I was initially thinking that it might be coming from my gas tank when vapor pressure built up if my tank was low on gas. But, that wasn't the case either. It will happen whether I have a full tank or almost empty tank. . . Just not consistently. It is very annoying and I hope others write in to complain about it as well.