High hard drive fan speed after SSD upgrade

I just upgraded the drive on my 2009 iMac to a 500GB SSD using the iFixit kit. Everything is good but the hard drive fan now runs at full speed all the time which I gather is a common problem after SSD upgrades. I am temporarily managing it by overriding system fan control using TG Pro to keep the fan speed down but doesn’t seem like a good, long-term solution.

I re-used the adhesive temp sensor from the old drive (now sticking to the metal case of the new SSD) and TG Pro reads ~40°C (@ 1600RPM). So, if the sensor is reporting that the drive is at a reasonable temp, why does the computer think it needs to run the fan so fast?

Is there a solution to this that lets the computer properly regulate fan speed without being overridden or is there some hardware flaw I need to address?

thanks,