The light weight quadcopter and hexacopter UAVs that can cary say 2kg to 20kg payloads for "delivery" produce enough noise that a steady stream of pizza and book deliveries might not be so welcome in many populated areas.
Are there any candidate technologies that can significantly reduce the noise heard by people on the ground? One possibility is of course to fly higher, but that may not always be possible or desirable, and I don't think they'll be deploying pizzas and books with little parachutes from 500 or 1000 foot heights.
Better propeller design? Cleverly ducted/muffled thrust? Anything?
I'm sure possible techniques will interfere with thrust, so anything that has an impact of less than 50% would count here. Batteries will continue to get better over time, but I can't imagine much more innovation can be done with propellers.
I'm not looking for guesses, but it doesn't have to be proven on this scale either. My feeling is that there really isn't anything besides distance that can help very much.