Small pump sound from underneath

So here's a new one to me. I went out at lunch today at work and i had just changed my diff fluid so i peeked my head underneath to check for any small leaks. From underneath the truck, mid to rear area, there was a small pump sound. Anybody help me out? This was as the truck was sitting for 4 hours, no key in ignition.

 

Perfectly normal. From the manual:

Approximately five hours after the engine is turned off, you may hear sound coming from under the vehicle for several minutes. This is the sound of a fuel evaporation leakage check and it does not indicate a malfunction

 

you guys never disappoint, thanks.

You know, im one of those people who read the manual too, missed that though.

 

It got me too. I must have looked like a moron laying under the truck in the dark poking at stuff the see what was moving.

 

Ha Ha, I had the exact same experience going out in my garage late one night and my truck is making noise. i was just about to post something up and saw this thread.

 

What if that sound keeps going on for hours? Is that a problem?

 

I'd think so... the test it's doing only takes a few minutes. Wonder if you've thrown a trouble code.