DJI has made a shocking amount of progress at making its drones quieter over the past few years. What used to sound like a loud swarm of bees 20 feet overhead is now almost imperceptible.
It’s not just the volume of air but also the speed that it’s moved at. Large slow propellers are quieter and more efficient than small fast propellers. (But less maneuverable.) In the limit, I could see this being done by big, spidery drones with huge props relative to their weight, like a scale model of https://en.wikipedia.org/wiki/AeroVelo_Atlas.
One thing the news release didn’t cover was whether there were attempts to integrate sound production into the coils of the motors.