Area 51 System Fan

Sometimes while gaming or just surfing the net, my system fan jumps to warp speed and stays that way for several minutes. I've used HWMonitor to check the temps when this happens and they look ok, highest core temp is core 0 at 49c. GPU is between 58-60c so it doesn't appear to be a heat issue. I'm running Win 7, i7-960 with 6 gig ram, Hd 5970 vc. Any ideas would be greatly appreciated as it gets loud and annoying with the fan running that high.