For a basic understanding about noise problems generated by wind trubines go the following AEI Wind Turbine Noise report 2009
The problem exist with most who visit a wind farm (usually during the day & usually in good weather conditions) and do NOT here any perceptible noise.
Noise complaints revolve around several factors
1) weather conditions which involve cool calm nights
2) low frequency levels which are not audible but are still perceived by your body (especially your inner ear)
3) air modulation which occurs when the blade passes the mast and creates an audible “swoosh”
4) harmonic power distortion or dirty electricity that is invading homes.