CPU fan intermittently loud / reporting 0 rpm

[wiimixer]

Does anyone have any experience / advice with the following situation:

Very old (first gen. Intel) iMac, running OS X 10.6. Recently developed an issue where the fan noise is distractingly loud. Using Mac Fan Control to help diagnose the issue, I discovered HDD & ODD fans appear normal, but CPU fan is 99% of the time reporting 0 rpm - and very loud. The other 1% of the time MFC reports a sensible figure, say, 1000 - and there is no obvious fan noise.

I can't find a copy of Apple Hardware Test utility to help diagnose, but on the basis of the info. above can anyone offer any advice - presumably CPU fan or related sensor is on the way out. Possibly detached and feasibly just needs reconnecting?

Thanks for any help / suggestions.
wiimixer