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What's the loud continuous noise the plane is making when it's on the ground and the (jet) engines are visibly not running (spinning, at least) ?

I was boarding and I thought it was perhaps a compressor for the cabin air but when I entered the cabin it wasn't cool inside.

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That would be an Auxiliary Power Unit, a small jet engine that is used to provide power on the ground when the engines are off and when there is an emergency in the air.

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    $\begingroup$ So, in other words, while the jet engines are visibly not running, an invisible jet engine is running :-P $\endgroup$ – Jörg W Mittag Jul 27 '16 at 22:39
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    $\begingroup$ I wouldn't call the APU a "jet engine". It's a gas turbine but it is designed to provide power (pneumatic, electric and/or hydraulic) not thrust. $\endgroup$ – Peter Green Jul 27 '16 at 22:45

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