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A couple of weeks ago I flew with SAS on one of its B737 (I believe -600 or -700). A few minutes before pushback and engine start, a rather annoying, high-pitch sound began - imagine a whining electric motor. At first I thought it was the sound related to closing the cargo door and I expected it to stop after a few seconds. It didn't, and then I began wondering whether it was something related to the APU. The sound remained the same even after engine start - and indeed for the remaining of the flight, though the engine sound somewhat covered it (or at least I got used to it, I don't know).

A uniformed pilot was also in the passenger cabin and he didn't seem at all perturbed, so I assume the sound wasn't anything out of the ordinary. And yet, I've been traveling for decades and I can't recall ever hearing this before.

Any ideas what it could be?

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    $\begingroup$ Where were you seated? Back, front or middle? $\endgroup$ – GdD Jun 20 '16 at 10:35
  • $\begingroup$ If you were sitting over the wing, it probably was the hydraulic pump. $\endgroup$ – SMS von der Tann Jun 20 '16 at 10:39
  • $\begingroup$ It could be the sound of the circulating fan. A few minutes before pushback, the plane switches from ground power to internal power. Same for air-con (on the ground, it is provided a pipe connected to the exterior of the aircraft). $\endgroup$ – kevin Jun 20 '16 at 10:40
  • $\begingroup$ I was indeed seated over the wing, a bit in front of it - come to think of it, it's a place I rarely pick. $\endgroup$ – Digital Dracula Jun 20 '16 at 15:16
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Well, expanding off my comment above, I was boarding a flight with Delta and the pilot said over the PA that the people sitting over the wing should not be concerned about a whining sound that they would hear in a few minutes as it was the hydraulic pumps working.

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To me this sounds like the ACM's (or airco-packs). In the B737, these incorporate a compressor and turbine, that might have produced the whining noise you heard.

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