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?
PGNCS From United States of America, joined Apr 2007, 2516 posts, RR: 45 Reply 3, posted (2 years 8 months 3 weeks 5 days ago) and read 3241 times:
The drop of ambient noise you refer to are the packs being turned off for engine start. The remaining background noise is a combination of other things and varies depending on where you are located. Most aircraft still have gasper air on at this point, and some may or may not have recirculation fans on as well (the 757 will have one recirculation fan on at a minimum due to the electronic cooling configuration of the aircraft.) There are also other electronics and electronics cooling components, hydraulic pumps, pneumatic components, and other contributors to ambient noise. I can't generally pick out single components though some mechanics no doubt can. The bottom line: I don't think the background sounds can be attributed to a single component, rather an array of items working simultaneously mostly under your feet.