Okay, so I got home tonight and the same scenario happened. The pulsating whine came on when I got out of the car. I unscrewed the gas cap and all the pressure came out and the whing stopped. I then stood there for about 30 seconds and the whining came back. I uscrewed the gas cap again and released more pressure and it went away. I am completely new at this so please be patient with my seemingly annoying questons. Should there be so much pressure? Is this normal?

 

Yes, there is pressure inside the gas tank and the fuel lines that is caused by the fuel pump. The fuel pump is taking the gas in the tank and pumping it from the tank to the engine. The gas takes up space and volume and when it is being extracted like that, there is no air taking its place in the gas tank. Because of this, it will create pressure. Think of when you were little, or if you are like SprintMax and you still do it, think of when you sucked in the air from a coke plastic bottle. The pressure "collapsed" the bottle. The same is with your gas tank. Except, for everyones case thank god lol, your gas tank will not flew like that. The actual pressure in the tank is not that much, it is greater in the fuel lines. I would say that the pressure in the tank should only be about 5-10 psi AT MOST.....

 

I think you shouldn't open your gas cap when the car still running.It will trigger a CEL.

 

my car makes venting noises as well when I shut it off
no biggie.