Note: (screenshots are provided for Mac but remember that Macs Fan Control also works flawlessly in Boot Camp Windows XP/Vista/7/8!)
Everyone who replaces HDD in most of iMac models faces the problem that HDD fan will start to spin at maximum speed after the replacement. This speed is about 6000-7000 RPM and the noise produced is unbearable (it happens because Apple puts special firmware in its HDDs)! What is more - you cannot just switch off the fan or put it rotating at MIN value (actually it's OK but for SSD), because it will cause HDD overheating.
If you replaced your HDD with some SDD - just set fan rotating at minimum speed (normally about 1000 RPM):
If you replaced your HDD with another HDD - you have to set it rotating in relation to HDD temperature:
These values (43C & 54C) are provided as an example for one of Seagate HDDs. One should referer to technical manual of his HDD and his preferences for noise/perfomance ballance.