"When it comes to noise the NH-U12S is not very loud at all. I’m sure you’ve heard so coolers that sound like a jet engine taking off! The NH-U12S is not one of those. The fan runs at only 1500 RPM and when using the low-noise adapter it goes down to 1150 RPM. Even when you add a second fan the cooler is not loud at all."
Bob Buskirk - Think Computers.
Noctua NH-U12S cooler review by Think Computers - Introduction & Conclusion.