Buzzing noise with engine off.

The other day I went out into my garage to have a smoke and I heard this buzzing noise that was coming from the back end of my Tacoma. It sounded like it was around the gas tank or rear end. I started the truck up with no problems and shut it off and haven't heard it since. This hasn't caused any problems but am concerned about what it was. My best guess is the fuel pump but the truck was off and no keys in the ignition so I don't know. Any idea's?

 

Fuel system check , normal

 

Can you tell me more about that? My truck was sitting for a couple of hours and I don't know how long it was buzzing before I went out to the garage. What is a fuel system check?

 

Thanks