there is a setting in the BIOS that has 2 choices.
fan on all the time
fan increases speed when under load
i have a scaleo P and the standard CPU/Heat sink fan was crap.
i got a different heatsink and fan and the noise went away
you _could_ clean the crap off the plastic fins (that is what is causing the vibrations and thus the noise)
or pay about £20-£30 to get a better heat sink and fan