I sounds like there could be something stuck on the fan such as a leaf - to check if it's anything to do with your air-con just switch it to economy mode and just use the fan - if the noise stops then it's something to do with your air con unit, if the noise is still there then turn the fan speed down and see if it stops, if it does stop or the noise gets quieter then you know it's something wedged in the fan.
if it is something like a leaf stuck in the fan try leaving it on full blast for a while, otherwise you may have to get the screwdriver out if you can't live with it!