Wind power

I passed very close to two wind farms on the way from Bremen to Bremerhaven, and it was interesting to note the contrast in noise pollution - from one all you could hear was a gentle 'swoosh', with the other emitting a grating sound which I assume was due to the gears. (I think the first must have been gearless, but I need to check these assumptions with an expert.) I wouldn't want to live too close to the latter, but having said that, the sound was drowned out by the noise from the road nearby, and a few hundred metres away I couldn't hear it at all