You could try re-pasting them with some thermal paste like the Arctic Cooling MX-4
One of my 760 windforces had a similar thing after I dropped a screwdriver on one its fans. The fan can be a little off centre and make a racket as they ever so slightly come close or just touch the heatsink. Place your finger on each fan to slow it down to a stop so you can isolate which or which ones are noisy. Move em around a little with some force. If it is just one fan you could just give it the chop and adjust the fan profile.