Even if the motor is quiet you still have the propeller making a lot of noise. I (honestly) don't know if you can really get it quiet enough for the motor's noise to be important.
The largest source of propeller noise is transonic flow at the tips, so turning the propeller slowly helps significantly. The tradeoff is a larger propeller and more expensive engine installation due to the required gearbox.
http://en.wikipedia.org/wiki/Lockheed_YO-3
The largest source of propeller noise is transonic flow at the tips, so turning the propeller slowly helps significantly. The tradeoff is a larger propeller and more expensive engine installation due to the required gearbox.