fuel pump whistle?

I noticed my car was making a whistling noise a while back. First noticed it when I was looking under the car with the engine running. I have all the back seats and everything stripped out at the minute and the noise is very apparent. Only way I can describe it is a high pitched whistling/hissing noise... like if a bottle of pop is opened ever so slightly and the gas starts hissing out?

It starts when the key is put in the primed position, just before the last turn to start the engine, when all the systems come on line and I'm pretty sure its coming from behind me? Not sure but I also think it changes pitch very slightly with speed/gear etc...

Is this the fuel pump or something? Any ideas what it is and if theres a cure? Not sure if it will go away if I put all the seats back etc but wanted to check if it was a problem or something that needs attention fist?
 
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