What is this sound from?

[GymB]

Toyota 4runner.
Tank is half empty.
With the truck running remove the gas cap and you can hear the engine running from the fill tube. Sounds like it’s maybe drawing air.
Turn it off, fill it up, turn it on, sound is gone.
Will try to post video in a few min.

 

[Pogue]

Evap emissions system. Don’t open cap with motor running. Will likely throw code.

 

[NCFubar]

Evap emissions system. Don’t open cap with motor running. Will likely throw code.

Some wise ass has been known to do that as a joke before … it can take a far amount of time of driving to clear it (just disconnecting the battery only temporarily does it on some vehicles).

 

[NC Boy Scout]

You could be hearing the fuel
Pump running too. They are inside the tank.

 

[NC Boy Scout]

Some wise ass has been known to do that as a joke before … it can take a far amount of time of driving to clear it (just disconnecting the battery only temporarily does it on some vehicles).

I had someone steal my gas cap at work once. Inside a parking garage in downtown Charlotte, in clear view of the security shack.

 

[GymB]

Evap emissions system. Don’t open cap with motor running. Will likely throw code.

Thanks.
Too late for that.
Daughter’s car and she hasn’t complained yet, so maybe I got away with it.
I have never turned a car off to pump gas, and I have never heard this sound before.

 

[Pogue]

Evap system opens a valve between engine intake manifold and the tank to draw vapors into charcoal canister. So there’s a vacuum in the tank. There’s also a pressure sensor in the tank that monitors the amount of vacuum. Sudden loss of vacuum usually results in a “large” Evap leak code. The aforementioned valve isn’t always open, and depends on vapor levels so you might here it sometimes and not others. And yes, you might could’ve heard the pump running but if so that’s a warning sign. At half tank it should be fully submerged and your more likely to feel a slight vibration/humm than you would be to hear it running. Loud pumps are usually failing pumps.