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?