Humming/Whirring Sound Under Engine

Hello,

I have a 2002 Mazda Tribute that I bought used.
I have been LOVING this vehicle especially since it is the LX option and therefore has the 4 wheel drive option.

Having 117,000 miles on it of course I was definitely expecting to have some issues sooner or later with it. After all it IS mechanical haha.

Anyway, because it is hard to explain I am also putting a link here so you can hear what I'm hearing and WHEN I'm hearing it as well.

http://youtu.be/XwcMI-Cki1Q

I can best describe the sound as whirring and it happens for a split second when simply turning the key to the on position before even starting. After starting the vehicle the sound starts again and with a very noticable and continual whirring or whining sound. The sound will disappear once and a while while driving but almost always comes back. It will completely stop once I get over about 30 mph but will return once I start going slow again near 20 mph or so.

When I get under the car to listen, the sound is definitely in the back area of the vehicle a little in front of the back wheels.

Any help with this problem would be most appreciated thanks in advance!

Fuel pump! When you turn the key on but not start the engine, the fuel pump turns on and then stops.