They're technically silent, but they will make a clacking sound when they bottom out, and when they pop back up. If you keep your finger on the key and slowly press up and down, you'll hear that the switch makes no sound. They feel really nice this way, but it's difficult to keep from bottoming out because they're so soft.
In practice, I usually bottom out, so they are noisy. When I bottom out, it's as if I smacked my finger into the backboard. The shock goes right back up my fingers. Still, I type on it all day everyday and it isn't that bad.