Depends, how fast can you run from our future robot overlords. Point being, noise should be the the least of concerns http://rt.com/news/167168-riot-control-pepper-spray-drone/

Well, if they are quieter, we won't hear them coming to riot control us.

Quick: http://www.wired.com/2012/07/owl/

That article is from almost two years ago. Use ducted fans, find ways to generate more lift at slower prop speeds.

That article says they are trying to make military drones "about equivalent to a chainsaw when up close".

So that's still louder than a lot of the hobby drones out there today. Having flying chainsaw-level noises everywhere will still be pretty annoying.