I'm not sure if it's the optical drive fan

After running an apple test from start up disc, I heard a fan kick in and ramp up to high speed, heard a snap and a horrible screeching sound that continues all the time when powered on. The computer runs fine. I have a clamp at the bottom right corning about 3.5" up and 2" over from right edge and this stops most of the noise when clamped pretty hard. I'm afraid of cracking something. I have smcFancontrol program running and when I change the ODD fan speed I notice the screeching change pitch. So is it a good guess I need to replace the Optical drive fan?

Hope you can help!

Thanks!

1.8 GHz EMC 2104 iMac intel 17"