Edit: I'm beginning to think this is more suited to the physics stack exchange, but I'll keep this here for a while...
Being bored mindless, surfing the interwebz, I came across this video, where a Sukhoi SU-57 is performing aerobatics:
At timestamp 1:45 the plane makes a pass, emitting a distinct screech with prominent components at about 1000, 2500 and 3600Hz.
This screech may very well be from external "add on" equipment such as radar reflectors (see this Q&A), as I kind of combine such noise to some kind of "fault" in the aerodynamics, and therefore I think during design phase sources of such disturbances in airflow would be eliminated from the airframe.
As first hand observations from real life, two examples: sailplanes and the B757, close encounter of which I have found to be memorable: the sailplanes for being very quiet, emitting a slight whisper with maybe a dash of a faint whistle at a high speed fly-by, and the B757s making a very distinct and sharp hizz on short finals (B757 is categorized as "heavy", not due to its size or mass, but for the nasty wake turbulence it makes).
So: can one draw conclusions about planes aerodynamic efficiency based on acoustic observations, and if so, to what extent?