737 sound

Quick question, in this video (between 0:01 and 0:15) what is the sound heard. I do not know wether it is the APU, Fuel pumps or the hydraulics. https://youtu.be/sqnr8X7AEF8

Thanks, Jacob πŸ‘πŸ»

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Probably the APU as they have that on up until engines are on… as well, you don’t really hear the full pumps, as they’re not that loud to overwhelm the cabin, so my best guess would be the APU, and then once both engines are on, they shut the APU off and the rest would be the engines

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Probably the APU? If not then probably the sound of the fuel pumps/engines starting up.

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