They're certainly not silent, but unless you're actually standing right under one they're rarely particularly noisy. Modern ones should be about 35 decibels at 300 meters distance. You're never going to hear an offshore one, really.
Yeah, under most circumstances you're really going to struggle to hear much, because if they're going fast, well, there's a lot of wind, and _wind_ is noisy, too. They make _detectable_ noise, but under normal conditions it's not really going to be human noticeable noise.
