'96 Safari "Jet Engine" Sound

[Jamykids]

We're looking to buy a used '96 Safari. (AWD, LT) When it's running, it sounds like a jet engine. You know the high pitched whistle kind of sound it makes as it's driving around the tarmac waiting to hit the runway?
Any ideas of where this sound is coming from? Hubby thought perhaps near the fuel tank. But you can hear it no matter where you stand as it's pretty loud. The seller says it has always made that sound.
Help?

[B.A.F.]

Sounds like a fuel pump. Tale off the gas cap and put your ear near the pipe.