After researching the blades a little further is seems a company called eurocopter did find a way to reduce the noise made by the propellers [1]. Heres a video of the difference between the two blade types [2]. Do you think this would be applicable to the blades at the drone level?
[1] http://www.wired.com/autopia/2010/02/eurocopter-moves-one-st... [2] http://www.youtube.com/watch?v=dBS1NRsYuF8
Airplane and drone props are entirely subsonic. They make a lot of noise when the pitch is significantly different than the speed that the airplane "should" going at. In other words: if you imagine a propeller cutting through butter, there is a certain distance it would go in one rotation. Propellers are loud when they're going significantly over or under that distance. Also - I'm just a hobbyist, so my explanation may not be correct.
This effect is especially noticeable with fixed-pitch propellers used for hovering.
For reference, check out these videos. Note that airplanes have 1D variable pitch.
[1] http://youtu.be/e8nYZ-fMSHg?t=1m5s - note the sound at 1m12s
[2] http://youtu.be/QB6vtE3rIrQ?t=9m15s - note the sounds at 9:25 and 9:53.
For more examples, see https://www.youtube.com/results?search_query=propeller+taxii...