Sorry, was a little bit too hasty. Just read this:
they are sometimes loud by design (i have seen some with 6000RPM fans there), or the fan could just be dieing, i would recommend you just replace the fan, something like this fan should be good, do NOT just unplug the fan, its there for a reason, and you stand a good chance of breaking something if the fan is off (and a lot of motherboards will just refuse to boot if you unplug the fan)
Now I'm confused, does the fan have a real purpose or not? My case is always open, so I don't think ventilation is the issue here
@ apache_lives: it is a MSI 875P Neo-FIS2R mainboard with Intel Southbridge. Which apparantly needs a fan too cool it (?)
Ack i was right it was MSI and it is the northbridge, and its an 875 chipset, DITCH THE FAN, it will work without it aslong as (already said) theres plenty of ventilation (which btw with most *decent* cases, will perform better with the side on, to remove air), and the cpu fan unblocked (no dust blocking).