"it is possible to run this fan at reduced voltage to make it unaudible. Running with the provided RPM-reducer NF-F12 PWM was absolutely silent and you couldn’t hear the noise of it under all the other noise coming from your computer. Also the rubber feet attached to fan will reduce resonating noises from the fan. I’d suggest this fan to anyone who needs silent, yet reasonably powerful fan in their computer with either CPU-cooler or general case fan."
Ville Ilonen - RBMods
Noctua NF-F12 PWM 120mm fan review by RBMods - Introduction & Conclusion.