There is a sensor cable on some iMac models that must be attached to the replacement HDD for the fans to operate normally. I do not recall if the particular models include your mid-2010 iMac but the symptom you describe would be consistent with this requirement.
If you did not buy it from OWC / MacSales or other well-known Apple supporter, it is unlikely to have the required hardware.
Check with OWC to determine the correct replacement you require.
'''It's an issue in your lower slot'''