Not long ago I was down below reading when I was disturbed by the whirring droning noise of a wind generator. Went up on deck for a look around and didn't see a boat with a wind generator. Oh well, I figure, the sound must be carrying through the water.
Turns out the noise was a vibration from my main halyard, led to the end of the boom as a topping lift. Slacked things off a bit and the noise went away.