Mysterious noise

[USMCASA]

so last night about 3am i was taking my dog out and noticed my 4runner was making a faint noise. sounded like a compressor but was very quiet almost like a aquarium pump or filter. it was coming from the drivers side rear tire well area. i went back inside to get my keys to check it out but when i came back out it wasn't doing it any longer. i know this noise wasn't present when i parked it around 1130. any ideas? could it possibly be the gas tank equalizing the pressure?

 

[dhylander]

so last night about 3am i was taking my dog out and noticed my 4runner was making a faint noise. sounded like a compressor but was very quiet almost like a aquarium pump or filter. it was coming from the drivers side rear tire well area. i went back inside to get my keys to check it out but when i came back out it wasn't doing it any longer. i know this noise wasn't present when i parked it around 1130. any ideas? could it possibly be the gas tank equalizing the pressure?

Here's a thread with the explanation.

http://www.toyota-4runner.org/5th-gen-t4rs/63923-noise-comming-rear-my-4runner-limited.html