I had a similar problem on my Geforce card. It was made by Guillemot. It turns out that it was a common problem with their fans. You can see it in their FAQ section. In fact, I am dealing with it right now. They originally said that they'd replace the fan. but then, they realized they were all out of fans and said they would replace the entire card. I haven't had it replaced just yet, but that's where it stands. It is likely one of your fan bearings is damaged and only a replacement will fix it. These fans are not common, so I suggest contacting the manufacturer.
reaper