Fans on IMac constantly

Please reset the NVRAM 3x back to back, if the problem persists then you likely have a hardware issue. However, there also may be something installed on the computer that is causing the problem if the NVRAM resets do not work. If the problem continues after the NVRAM resets please do the following:

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate towww.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click https://discussions.apple.com/docs/DOC-250000211 It is helpful to us if you download the report to your Desktop.

...the fan is continually running. It is running about 96% idle and is not warm. Fan vents are clear, I have reset SMC - 3 times...

Fans are on all the time. They speed up when temperature sensors detect increasing internal temps. My 2017 iMac 5K's fan idles at 1200rpm— just audible— and its max speed is 2700rpm and very noticeable.

However, should a temp sensor fail, that will trigger the system to set the fan to full speed to protect itself. That is a safety or "fail-safe" measure independent of actual temperature, and fits your description of "not warm."

Continue with EtreCheck, but it will not pick up sensor errors. Apple Diagnostics is very good at detecting bad sensors and you can run it at home. Instructions here:

Use Apple Diagnostics to test your Mac - Apple Support

If AD throws out an error code, look near the bottom of that article for a link to another article that translates AD error codes into plain language.