Loud fan, hot top left edge

I have an iMac 21.5" from mid 2011 that I don't quite know what to do with.

On startup, and initial boot, it's fine. After a while though, a loud fan-like noise occurs. Given enough time, the top-left of the iMac near the ventilation strip gets hot to the touch. It seems to be coming from close to the top of the iMac, so I thought it was the hard drive. I replaced it, thinking that might be the solution. (it wasn't but it can now run MacOS Sierra reasonably well, so I'm pleased with that)

I took a closer look, hoping to diagnose it further. I installed iStat on the iMac, and it showed that all internal temperatures were hovering between 97-148 degreed Fahrenheit. At this point, the fan(s) was (were) running quite loudly.

I poked around a bit on the interwebs, and came up with two possible solutions:

-Add a Digital Thermal Sensor

-Replace the CPU fan

Just looking for some feedback as to which course of action might be preferable. If more info is needed, let me know.

Thanks in advance.