Recently my 27″ iMac has been generating quite a bit of uncustomary fan noise – even when practically idle! After downloading and installing
smcFanControl, I found out that the problem was my hard disk drive fans running at excessive speeds of 4500rpm!! If you are facing the same noise problem that I was, this is the solution:

1. Ensure that you have installed smcFanControl (freeware)

2. Launch Terminal, which is located in your Applications – Utilities folder and type this command

/Applications/smcFanControl.app/Contents/Resources/smc -k F1Mx -w 2af8

This now sets your Hard Drive fan speed to 2750rpm!
If you are certain that the ambient temperature is low and you aren’t running anything CPU intensive, you may want to get the noise (?) levels even lower by replacing 2af8 with 12c0 i.e. 1200rpm!