Inversion, try this. I had the same problem and i emailed Creative LAbs to try to get a replacement fan. They told me to add some oil lubricant to the motor of the fan.
To do this, just take off the fan and FLIP it over. Then peel off the sticker. Under the sticker is a very, very, very tiny small white hole and that's where you place, one very, very, very tiny drop of your regular household oil lubricant. Then put the sticker back on and flip the fan over and screw it back onto the heatsink.
BTW, if you can't find the white hole after peeling off the sticker, then you peeled off the wrong sticker on the wrong side. Just try peeling off the sticker on the other side.