Humming from rear differential while truck is off?

Just got my truck back from the dealer after they replaced my front passenger axle and differential.. drove home, my truck has been parked for 4-5 hours.. went to leave and I heard a humming from my rear diff. Now like I said the truck has been off and parked for 4-5 hours. I never heard this before and I’ve owned the truck for 2 years. It basically sounds like a humming transformer

 

It’s a smog pump. It’s in the owners manual.

 

The 3rd gen's evaporative system makes noise roughly 4-5 hours after being turned off. I would suspect this is what you are hearing which is totally normal.

 

Most likely the fuel vapour recovery system pump. This is normal.

 

As the other guys said, normal.