what is this sound? i only hear it when car is on. I really only hear it when im outside the car. It seems to come from the gas tank. Removing gas cap doesn't stop the sound.

 

Turn the key in the ignition as much as possible without starting the car, and listen to see if you can hear this same noise for about 1-3 seconds before it shuts off. If so, it's just the fuel pump. I don't think the stock pump should be this loud, but maybe someone else can chime in who's changed a few out.

 

Thanks I haven't done your test yet but I'm getting Fuel Supply System CEL. I made a separate thread about it. Could this be related