Designing for Low Aerodynamic Noise from Wind Turbines

Aerodynamic Sources of Sound Emission
When the wind hits different objects at a certain speed, it will generally start making a sound. If it hits the leaves of trees and bushes, or a water surface it will create a random mixture of high frequencies, often called white noise.
The wind may also set surfaces in vibration, as sometimes happens with parts of a building, a car or even an (engineless) glider aeroplane. These surfaces in turn emit their own sound. If the wind hits a sharp edge, it may produce a pure tone, as you can hear it from musical wind instruments.

Rotor Blade Sound Emission and the Fifth Power Law
Rotor blades make a slight swishing sound which you may hear if you are close to a wind turbine at relatively low wind speeds.
Rotor blades must brake the wind to transfer energy to the rotor. In the process they cause some emission of white noise. If the surfaces of the rotor blades are very smooth (which indeed they must be for aerodynamic reasons), the surfaces will emit a minor part of the noise. Most of the noise will originate from the trailing (back) edge of the blades. Careful design of trailing edges and very careful handling of rotor blades while they are mounted, have become routine practice in the industry.
Other things being equal, sound pressure will increase with the fifth power of the speed of the blade relative to the surrounding air. You will therefore notice that modern wind turbines with large rotor diameters have very low rotational speed.

Rotor Blade Tip Design
Since the tip of the blade moves substantially faster than the root of the blade, great care is taken about the design of the rotor tip. If you look closely at different rotor blades you will discover subtle changes in their geometry over time, as more and more research in the area is being done.
The research is also done for performance reasons, since most of the torque (rotational moment) of the rotor comes from the outer part of the blades. In addition, the airflows around the tip of rotor blades is extremely complex, compared to the airflow over the rest of the rotor blade.

Research on Quieter Blades
Research on quieter rotor blades continues, but as mentioned in the section Noise is a Minor Problem, most of the benefits of that research will be turned into increased rotational speed and increased energy output, since noise is generally not a problem per se, given the distances to neighbouring houses etc.