What is this above the gas tank?

Above the gas tank but below the bed, toward the rear, is a box a bit bigger than a couple cigar boxes end to end. The 2 rear wheel sensor wires enter it, so do some other stuff. What is this?
Anyone?

Mine started making some noise when the truck was turned off.

 

Probably the evaporative recovery unit for you emissions system

 

I have to do some research, wish the digital manual was easier to search. I'm concerned that there is an active internal component that turned on with the key off and removed from the ignition.

 

Mine did that one time too. Middle of the night with everything off. Went out to garage to have a smoke and the thing started buzzing for no reason. Never heard it again.

 

Big ol' box of charcoal and a pump that costs 1000 bucks.
I know this because the lot attendant who filled my truck up when I bought it new topped off the tank which ruined the box and threw a CEL as it was being driven home.

 

Found an explanation on p8 of the owners manual. 5 hours after shutting off the engine, the truck performs a self test.
I don't like it, but there it is. Another phantom electric drain on the battery.

 

And he no doubt thought he was doing you a favor!