This is probably a stupid question, but assuming some noise pollution is inevitable, is it plausible that a motor could be tuned, or have its noise output "converted" in some way to emit a pitch higher than the frequency range audible to human ears? Or even a lower frequency that was more in line with the existing noise pollution we're accustomed to.
Edit: maybe we could RL a somewhat efficient ornithopter. A mechanical harpy eagle with owl-like noise dampening serrations on its wings would look way cooler flying through our cities and be much quieter.
Simple answer: drone noise is a combination of narrow band and broad band noise. The narrow band frequency comes from the RPM of the rotors spinning, but the broad band noise comes from the turbulence inherent to propeller lift production and the frequency cannot be contained to above/below human hearing.
The overall magnitude of the noise production can be reduced a bit by using lighter vehicles (less thrust needed) and more efficient propellers, but it's never going to be quiet unless we do away with propellers, which is very unlikely to happen.
Wing has already changed their drones to attempt to reduce the annoyance from the noise.
Better batteries may also assist to make drones quieter: if you have a bigger energy budget, you can afford to waste some of it using quieter but less efficient propulsion (like those toroidal rotors discussed here a little while ago).
Edit: maybe we could RL a somewhat efficient ornithopter. A mechanical harpy eagle with owl-like noise dampening serrations on its wings would look way cooler flying through our cities and be much quieter.