Researchers have figured out how make shaped drone rotors that convert audible and ultrasound frequencies into stable mechanical spinning motion, leading to tiny drones that can be completely silent. While scary-sounding (no pun intended) it’s unclear how usable these would be in the real world, since a line-of-sight source of sound waves has to be provided.
Of course it’s silent. It’s doubtful our sonar would pick it up, and if it did, it would sound like whales humping, or some kind of seismic anomaly