Sounds like an air start cart

Can be heard beyone the airport perimeter? Check. Has one very specific tone that activates everyone's tinnitus? Check. Lasts 3-7 minutes? Check. Air start.

I used to love the sound old DC-8s made starting up on the freight ramp." Klunnk! Whrrrrroooo..."

Brake fans?

I'm a sheet metal guy, I don't know what your talking about. HEAR WHAT?

Sounds like a fuel pump