So, this question has been bugging me for years.
Around 39 seconds into the video, the fans turn off and you just hear the hum of... what? What is it that makes that background ambient hum that's unique to the 757?
The 737 has a similar hum, perhaps a little more distinct. You can hear it in this video, although it is quite loud for some reason: http://www.youtube.com/watch?v=o1od3I9fsvM
I've heard that it's pneumatic systems that cause it, the "air packs" (which I'm still not sure what are exactly), the APU, so many different things. Does anybody know for sure what creates this noise, and how it does so?