Hi Boffy!
If you're sure that it's the CPU fan that makes the noise, then you could try one of these:
http://www.arctic-cooling.com/en/pro..._silent2r2_tc/
I had an Athlon XP 1500+ that sounded like an express train and after switching to one of those babies, I had to look at the power LED when I turned it on to make sure that it really was on.
Using a screwdriver and a lot of force to pry the new fan in place is pretty scary, but that's another story.
Martin