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.
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.