It shouldn't happen, but it does, quite frequently. I've had 2 or 3 cards over the years whereby i've had the fan die. It is usually on the larger fans, over 40mm, when they can degrade pretty quickly. If they are going to fail, it's usually at the 6-8 month mark with normal use, high use and your talking 3 months.
Sleeve bearings can't handle high heat, this mixed with exposure to dust means they can fail very quickly. The reason that larger fans fail is not just due to the fan's mass putting more pressure on the bearing, but because larger fans are put into hotter environments to push more air. Some models actually use a little bit of oil with the sleeve, and these tend to last longer. Most, especially the cheap end, are just simple polycarbonate washers put on either-side of the shaft, no lubrication at all.
If you're up to it (and the card is out of warranty), you could take out the fan, remove the sticker on the back, pop off the sleeve, pull out the fan and then give it a damn good clean, then use a little drop of engine oil on the shaft, and another drop on the sleeve. If the fan has been rattling for a while, there's a good chance that the shaft and fan has become imbalanced and this clean will only give you another few weeks of use, and it'll just fail again.
