I often wondered about that as well. What you hear is probably a combination of various noises. The two most likely are the airconditioning packs and the Auxiliary Power Unit (APU). Taking the airco. pack first, here is a
diagram. Air from the APU is bled off but it has to be cooled. An aircycle machine does this job. Both the inlet and outlets make a fair bit of noise. The
APU is a small jet engine (well gas turbine to be precise) and its job is to produce air, for airconditioning and engine start and electricity when the engines generators are not running. Not surprisingly, it sounds like a jet engine and again, the noise is generated at both the inlet and outlet.
Ailerons - at speed you hardly need any any control movement and spoilers may be used instead of or as well as 'up' aileron. And yes, some fly by wire aircraft automatically demand the use of spoilers (on the downward moving wind) in turns.
PM