iMac fan runs full speed on startup

Take it back to who ever did the work and have them fix it correctly.

Depending on which year model it is, either they forgot to transfer and reconnect the Temperature Sensor (in the 2006 - 2008 models) or put in an incompatible Hard Drive (in the 2009 - 2011 models).

If the fan always runs at high speed, the drive may not have been installed correctly, or it may be faulty.