Humming from rear at a stop

[Sifrk]

A couple of months ago I was camping out of the car on a back road. At about 1AM in the morning I was awoken by this low humming noise that I'd never heard before. At first I thought it was someone maybe down the trail running some sort of machinery ... sounded almost like a tow truck winch. Anyway, I opened the door, stuck my head out, but couldn't really hear it outside. After 5 - 6 minutes it stopped. Didn't give it much thought after that.

A few weeks ago, at the house with the 4R having been parked for hours. I was outside doing something and I hear it again. I localized it to the rear of the 4R coming from behind the bumper. Sounded like it was coming from the left side. I have no idea what it may be. Hoping someone here can help shed some light?

 

[Saxainpdx]

I'm guessing it is this:
http://www.toyota-4runner.org/5th-gen-t4rs/63923-noise-comming-rear-my-4runner-limited.html

 

[tm965]

I'm guessing it is this:
http://www.toyota-4runner.org/5th-gen-t4rs/63923-noise-comming-rear-my-4runner-limited.html

i heard this once, thought wtf is that... this car doesnt have an air suspension. i turned the car on and off again, and it was gone. this makes sense.

 

[Sifrk]

I'm guessing it is this:
http://www.toyota-4runner.org/5th-gen-t4rs/63923-noise-comming-rear-my-4runner-limited.html

Thank you, sir!