Buzzing sound around gas tank

When I closed my garage tonight, I noticed a buzzing sound from my truck (hasn't been driven in over 7 hours) coming from somewhere around the gas tank. I turned the key to on, it stopped buzzing, turned it off and it didn't come back yet. Any ideas what that sound could be and if I need it looked at? My best guess is the fuel pump but it doesn't make sense to be on with no key in the ignition and parked for several hours.

 

Thanks! I should of done some googling before starting a thread...

 

I took my brand new Tundra for an impromptu camping trip the first night I had it, and slept in the bed. I woke up from a dead sleep in the early morning to a buzzing noise, it was coming from underneath the truck bed, right below my head.

Needless to say, I was quite baffled by the sound, crawling around under my truck half awake with a flashlight, not entirely sure if I was actually just hearing someone inflating an air mattress with an electric pump a few campsites away.

That was my introduction to the weird and unexpected noises a modern vehicle makes.

 

fortunately for me.....when you get older like me...you don't hear stuff like that...