While usually when a fan starts making a loud noise it means it's time for a new one, sometimes you want to extend it's life until you get a replacement without braking the bank - I remember I had to look for while a couple of years ago for a fan replacement for my Radeon 9600 XT card and I eventually found a Vantec Iceberq 4 on the web but it didn't come cheap at all. So now that that this one started to make a bit of noise, I wanted to make sure it keeps spinning for a bit longer.
I only discovered WD40 recently and I was so impressed with it that now I can hardly resist using it with anything really, but luckily I said it must be too good to be true and looking on the web I found this article on ZDNet explaining how computer fans work and what you should do.
Fix noisy computer fans with a drop of oil
Ask in your local hardware store for 3-in-1 oil or sewing machine oil. I found 100ml 3-in-1 oil made by the same WD40 company at 3.5 Euro at a my local Lenehans store.