Hmm.. so i've done a bunch of reading at various locations on the subject, and come up with the following:
1) Seems that 2009/2010 model iMacs have a common problem experienced by people that have upgraded their HDD's. The culprit perhaps seems to be that the cable used to connect the sensor is not fully/properly seated into the new drive. As my iMac has a 1TB drive, and this is the default configurations size for this model, i can't assume that the drive has been replaced causing this problem.
2) smcfancontrol, the app recommended in many locations that are discussing fan control, isn't really much help for the type of problem i'm experiencing. This app only controls the MINIMUM fan speed. Especially, it lets you raise (but not lower) the minimum fan speed higher than the apple default. Since my fan is running faster than it should already, would not help me.
3) There is an app called HDDFanControl. (
http://www.hddfancontrol.com ). It claims to use different sensors than what's used through that cable that isn't always seated properly. And it claims to help LOWER fan speed. Seems good. But the app is $35.. which to me seems a lot to not know for sure if it works. They have a free trial, but it's only a 1 hour trial. Yikes. I'll probably give it a go. If that don't work, i guess my new step is to try to open up the iMac and check out that HDD, and it's cable connection...
Update: I installed the HDD Fan Control app.. and the fan in the iMac immediately dropped from 3700 down to 1090.... and for the full hour trial, it stayed right in the range of 1000-1100, where it is supposed to be. I carefully watched the temperature of the cpu, hdd, etc... while i put it through a lot of usage during that hour. all looked good. But right at the end of the hour, it popped up a window letting me know my trial had ended... and the fan shot right back up to 5000rpm. Ugh. Guess i have no choice but to buy the app...
