Answer: A whisper.
Well maybe not in the literal sense, but it is when compared to the whine of drones with traditional propellers.
According to New Atlas, the Whisper Drone prototype was developed by electrical engineer Derek Schwartz, who was thinking about how loud the whine of traditional propellers would be when they were large enough to carry human passengers. His four ring-shaped propellers replace the whine with a quieter and pleasanter whooshing sound.
Schwartz’s design works well, though its not as efficient as traditional propellers — yet. However, he does believe that the polyurethane foam he used to make them is less fragile and more likely to survive a collision. The propellers can also be built using less material without compromising their strength, because they have low internal stresses.
The Whisper Drone can be heard in action on its Kickstarter page.