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 only for SSD), because it will cause HDD overheating.