Humming sound in 4LOW when truck is off?

Alright so I went camping over the weekend, needed 4LOW to get up to the spot.
Parked the truck and went to bed a few hours later, I'm laying in my tent and notice this weird sound, almost like a really quiet fuel pump or an inverter fan. So I got up and went out to my truck to check things out, key was out, everything was off. At the time I thought it was my fuel pump so I cycled my key and the noise stopped. This morning I'm drinking my coffee and hear the sound again, key out of the truck, nothing on, doors closed etc. I crawl under the thing and the sound is coming from somewhere up front near the diff. 4x4 works fine...well as good as it ever has I guess. I haven't heard the noise come back since the truck was parked in my garage in 2wd, not sure if it does it in 4 HI
Anyone else run into anything like this? Any idea what it might be? 4x4 actuator?

 

It will do it in any mode you leave the truck. It's the emission system self test.

 

4lo is a coincidence, it's the evap system pressure test.

 

Yeah my blow off valves make noise too when I shut er down. Its normal

 

Ah makes sense, just weird that I've never heard it when it's parked in my garage and I think I spend more time out there than in my house.

How often does it self test? It did it 3 times parked up there without me touching it, one lasted like an hour.

 

The longer you touched it the longer it will last

 

Not for me, it's usually over pretty much as soon as the touching starts.

 

Probably falls under "Just the tip" event.