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When flying in an Airbus, I often (if not always) hear a weird sound before the aircraft starts to accelerate for taking off. From where exactly does this sound come from? Why doesn't it appear on other aircraft?

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    $\begingroup$ Hm, I wonder what sound you mean. The PTU is heard when starting engines, so during or just after push-back. I can't remember anything before the aircraft starts to accelerate. Can you find some video that would have that sound? $\endgroup$ – Jan Hudec May 11 '14 at 16:38
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    $\begingroup$ It's definitely the PTU as you can hear on this video or this other one. And I also do remember hearing this sound at the point just before the acceleration for take off. $\endgroup$ – Ludovic C. May 12 '14 at 19:20
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    $\begingroup$ That's the PTU, the Passenger Terrification Unit, which is designed to terrify the passengers and quiet them down by making loud, banging noises. It comes on every so often and can also be manually activated by a button in the cockpit. $\endgroup$ – Tyler Durden Mar 16 '15 at 23:18
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According to Ask the Pilot:

Almost every frequent flyer has encountered this sound at one time or another. [...]

It happens on twin-engine Airbus models: the A320 series (includes the subvariants A319 and A321) and the larger A330. [...]

What you hear is a device called the power transfer unit, or PTU, which is designed to ensure adequate hydraulic pressures during single-engine operations. [...] Since it is activated only when the pressure falls below a certain level, the PTU cycles on and off, on and off, on and off.

[...]

Some Boeing aircraft also employ a PTU, but the operation is slightly different and it doesn’t bark like a dog.

Another noise peculiar to Airbus models is a shrill, prolonged whine heard at the gate prior to departure and again after landing. This is an electric hydraulic pump used to open and close the cargo doors.

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