That random noise after your truck is off...

Hi all.

recently noticed some strange noise coming from the truck after it had been turned off.... I mean like hours after it has been turned off.

The first time I noticed it was when I was sleeping in the back of the truck a few weeks ago. I was worken up by a humming noise that had started. no idea what it was got up out of the truck, started it up, noise went away, shut downt he truck noise went away, I went back to bed..
Second time I noticed it was when I was walking out to grab somethign from the truck several hours after it had been shut off, again it was at night.

Sounded almost like a small pump humming away, seemed loudest about mid way down the body of the truck

any ideas?

some context

2012 reg cab 5 speed 2.7 4x4

thanks

 

Smog pump no need to freak out unless your truck is throwing codes

 

as in linked to the good ole charcoal canister?

figured it was supposed to be doing it just never noticed it before after having it for a year thanks for the info