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.
I have some wind turbines near my town. I can walk right up to the pole, and I can't hear any noise from the turbine.
Light car traffic is ~50dB.
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.
They're most certainly not silent! If you're standing right under one, you can hear it over car traffic and the breeze! (Well, unless the wind is blowing really hard: then the wind might be louder than either, considering the usual minimum distance between wind turbines and roads.) Whatever will you claim is silent next? Refrigerators? Lightbulbs?