Truck humming while turned off

I was walking by my truck a couple of minutes ago, and I noticed a consistent humming coming from the rear. Not sure if it’s normal and I’ve just never noticed it or what. Trucks been turned off for 5-6 hours. Sounds similar to an air compressor, but just a lot quieter. Has the same noise and rhythm.
It’s not letting me upload a video, so I guess I’ll just have to hope someone knows exactly what I’m talking about…
Any insight?

 

Pretty sure it's the evap/purge system.

 

That makes sense being that it was loudest next to my fuel tank. I wonder why it’s so loud. It’s pretty windy out right now, and I was a solid 5-6 feet away from my truck when I heard it.

 

That I dunno, but I can tell you it's not much quieter inside the vehicle when it's off. LOL. My wife's 4Runner does it too, I believe.

 

Yep, ours does it frequently. Loosen the cap, it'll vent and stop. Not sure why it does it but it does.

 

It's a leak check. It's creating a vacuum to see if pressure holds. Safety and all that.

 

You don’t want to loosen the cap because it’s performing a test and that will result it it failing. Just let it do it’s thing, it’s normal.

 

When y’all say “loosen the cap” I assume you mean the gas cap*
Just went outside and it stopped. I gave it another 2-3 minutes and then opened the gas cap out of curiosity. Sounded as if I opened a soda can along with a gigantic puff of gas in my face lol. Glad I was able to get some answers without a video.