[adamweld]

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.