iMac late 2009 fan issue

I have an iMac late 2009, Intel i5 266 GHz. I've replaced CD drive with OWC SSD and made it boot drive. Old HD relegated simply to data storage.

Timing was perfect as old HD then died. I had to remove it as it was causing performance issues (was making a clicking noise and causing OS long delays when waking up). Furthermore, I noticed cooling fan (not sure which one) was kicking in after just a few minutes, spinning at max rpm and never stopping.

With HD removed the OS responsiveness has returned to normal but the fan issue has not stopped. I've opened up my iMac and checked that the heat sensor on the OWC is positioned correctly. I've also cleaned out dust from fans but the fan issue does not go away. Using my iMac has become sooooo noisy!

Could it be that in the absence of an HD in the traditional HD bay is causing the OS to be confused? Should I buy a new HD to replace the old one even though I don't actually need one? I cannot find any info online and simply don't know what to try next.

I know my iMac is old but it has maxed out RAM and with the upgrade to SSD boot drive performs extremely well with MS Office, internet browsing, iTunes, etc. I don't need to change it for performance reasons.

Thanks.

David