Spend $10-20 bucks on a new one(s). That is if its your case fans that are making the noise. CPU fans are a bit more for quiet ones. If you dont have heat issues, get an adjustable speed fan that you can turn down or set to auto monitor. I have pretty good flow in my case and no heat issues. My case fans are turned down to around 1200rpm and make virtually no noise. My cpu fan is a bit noisy, but I am eyeballing a couple replacements. Try this site -
http://www.plycon.com