You sure it hasn't got a punture!?!?!

Seriously, if the fuel pump was pulling in air, then it would be leaking too, and stink of fuel.
Is it when the engine is running, ignition on, or everything off.
If it's when its running, have a look behind the splash guard at the front of the wheel arch, where all the tank breather pipes run, maybe one has split and is 'whistling air'
kraM