As to addis's point of sleeping with the same "frequency" earlier in this thread, this isn't the same with all fans, all components that come with fans or fans them selves, come with a noise specification - this is either measured in Decibels (DB) or Sone's.
There's a big difference, my Heatsink & Fan a Artic Freezer 64 has a 1 Sone specification, basically the difference between DB and Sone's are that Decibels are the PURE measurement of sound, where as Sone is the measurement of sound as to how noticeable it is to a Humans ear..
Ie 3 Sones could be 8Decibels, but the frequency and pitch of it could drive you insane - (White Sound), where as you could have a computer that has a Max output of 55DB, but the fans being used could be 1 Sone, ie, the fans are very loud, but barable, merely because the frequency and pitch of the sound been made is not conflicting with any case chasis ie creating turbulance, or the ball bearings in the fans creating a scratching noise..
My 21Decibel Vantec Stealth Fans are quiet due to the fact i have removed (cut out) all the fan grills, and they still run at 21Decibels but they are quieter.. Sone wise.. ie to ME or any Other Human they are quieter.. due to less turbulence..