This is a (relatively) common problem. Setting up a custom fan curve will solve your problem!
This is a different GPU, but the fan curve bit is what you need: http://www.overclock.net/t/1265110/the-gtx-670-overclocking-master-guide
"Also pay attention for any annoying tone shifts. For example, if at 55% and 65% the fan sounds fine, but at 60% the fan makes a aggravating high-pitched squeal, then it might be best to completely skip that RPM when designing the fan curve. Here's an example:
You see how it completely skips 60% and goes straight from 55% to 65%? There is one draw back to this though, and that is the temperature might get stuck at 55C oscillating between the two fan points. So you would hear the fans constantly shifting from 55% to 65%. You will need to decide if this fan throttling noise is better than the pitch change. If you're lucky, you'll be able to optimize this a bit and set the fan to jump from 58% to 62% and bypass the problem RPM's without having a noticeable fan throttling sound. You'll have to test that for yourself."