Fans like to fail this way when the bearings are shot. They start spinning up, and there are some nasty grindy stiction noises and the fan slows down.
Since sending it back for an RMA (unless you get an advance replacement) will cost you bitcoins, one way that you can give the fan some more life (depending on the design) is to peel back the sticker or plug covering the bearing shaft, and put a drop of thin lubricant (like 3-in-1 oil, transmission fluid) on the shaft and circlip without making a mess, and then put the sticker back on. Spin it around a bit so gravity helps it penetrate. Fire the card back up and mine away.