I'm surprised that no one has mentioned HDD Fan Control. HDD Fan Control "HDD Fan Control works to fix this issue by reading the drives internal temperature using the S.M.A.R.T protocol and set the fans actual speed to a value good to protect the drive. It runs at startup and continually to always control the fan correctly, prevent the loud fan noise and protect the drive from overheating." Of course, if you're a tech, you wouldn't want to depend on the user to keep the software in place. I didn't know about this issue before I bought a new imac expecting to put in a ssd & a bigger drive & more memory. Still debating about sending it back and buying a used 8core pro before I tear in to it using the ifixit kit.
Seconded ... remove ram first! Spent more time reinstalling main board than anything else.
Added bigger HDD as well as SSD. Buy HDD Fan Control to fix fan speed.
A curse on Jobs & Apple for making it this tedious to add SSD & HDD.
Cudos to ifixit!