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　　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".