What is this part?

I walked into my garage and heard a noise coming from my truck a few hours after it had been driven. I snapped a pic of the part and I'm trying to figure out what it is. Sounded like a pump, maybe?
The noisemaker in question is the black/gray box top center in the photo. It's located above the driver side rear axle. Pic is looking toward the front from under the tailgate.

 

Whirring sound? It's doing a pressure test on the fuel tank. It does that on occasion. It freaked all of us out when we first heard it too.

 

^

 

Whirring, yeah. I'm relieved.
Nice feature, I guess, but uncommanded operations when the truck has been off for a couple hours are kinda freaky.

 

Pesky owners manual trying to help again.