Idiom:

Put your money where the mouth is ( support your words with actions )

Hit the book:

Humming noise

Auxiliary

Flicker

Whine

Hydraulic

Taxiing

Unnerving

Prolong

Flap

The humming noise when you board

That's the auxiliary power unit at the back of the plane. "That baby hums along while you are boarding the plane to keep every thing up and running while the main engines are off," says Leary. "In addition, most airports hook the plane up to the ground units to pump air and electricity into the plane to save money." When the doors close, you may notice the noise changes, and the lights will flicker, as the pilot switches from the airport system to the aircraft system. Next you'll hear the engine's being started – first "the whine of the spin-up" and then "the roar".