My right-bottom fan in my imac is making a bit of noise and need to know what its for

Yeah, coming from the generation of iMacs that seemingly has issues, I felt lucky to come out unscathed until 3 weeks ago, when my fan began to start making noise out of the blue, like its bearing is worn out (Just to mention, this is an iMac Mid-2009, 2.66Ghz, 20.5"). It comes and goes, irrespective of the temperature in the computer. Even as I write in made some noise for about a minute or two and left.

It runs Windows 7 (for 3 years and counting), given all the freezing issues I had with the Mac operating system in the final months before switching. And I hear that Windows might have a harder effect on the fans, though in my case I couldn't tell the difference.

I had some software installed 2 months ago which monitors temperature on both CPU and GPU after feeling how hot the iMac was. Since then, the peak temperature for both cpus has increased by about 10C (at about 50C), graphic card stayed the same at 85C (max), for some games/video conversion.

Fearing that there was too much dust in there, I began by opening the iMac (my extended warranty has already worn out) and vacuuming it out. However, there was hardly any in there. The left-bottom fan is just fine. However, spinning the right bottom fan showed that it was the one making the noise, so I am guessing it has to be replaced (I guess it helps cool the logic board and CPU?).

So, my question is what purpose that this fan serve and aside from replacing it with a refurbished version of it (that costs almost $85), is there a possibility of replacing it with a less expensive one, but of better quality (a normal case fan)? Essentially, if this one went out, I am guessing that the others will follow too and I am sadly not made of money just yet.