Owc temperature sensor switch :(
Repair was pretty easy except for the fact that after everything was complete it turns out I also needed a " owc temperature sensor switch " replacing the oem hd require this cable which connects to the hard drive and temperature sensor ...if not connect the fan thinks the hard drive is failing and speeds up on full steam and becomes extremely loud . 60$ spent getting there part now I have to take apart iMac again
Easy repair ,take your time but super easy 30 mins repair time 1 hr if u take your Time .
Pick up a owc temperature sensor cable of your doing a hard drive replacement