It is probably a bearing going out. Also, make sure that there isn't anything close to it that could get sucked in when it speeds up. Is this the stock fan that came with the CPU? Intel/AMD? How long have you had it? If it is stock I would greatly recommend a nice aftermarket one. Something like the Cooler Master Hyper 212 (http://www.newegg.com/Product/Product.aspx?Item=N82E16835103099) would do just fine, and they are nice and quite.