Abour two years ago, I built a new home PC for myself. I got a nice Antec Sonata case, and a MB with variable-speed fans to cut down on noise. When I put everything together, I was very happy with the sound level except for the stock Intel CPU fan, which idled quietly, but sounded like a vacuum cleaner when it kicked into high speed. I decided to upgrade the heat sink and fan, and decided on an Arctic 7 Cooler, which ran very quietly until the last few weeks.
I started to notice my PC getting noisier and noiser, and when I popped the side off, I could tell that the noise was coming from the fan on the Arctic Cooler. I just got done fixing it, though, so if you've got a noisy Arctic 7 Cooler, don't throw it out -- try some machine oil.
(BTW, in browsing for a picture of the Arctic 7, it looks like they've upgraded the fan in the mean time, so the problem and the fix here very likely only apply to the older units)
First, you'll most likely have to remove the heatsink in order to oil the fan. Next, look for a little plastic plug on the back of the fan assembly (see picture). Pop this off with a knife. Next, just send a few drops of 3-in-1 or similar machine oil down the hole and spin the fan a bit to work it in. That's it! Reassemble everything (you may want to freshen the heatsink compound if you overclock your CPU), and you should be good to go.
