with the car off, gently move it around if you can feel any sort of friction, even the slightest bit, it will cause your fan to vibrate and become noisy.
it wont harm your engine extensively, but it will put more strain on it, also your belts will experience the same thing.
i had a smililar problem at 5000rpm... i replaced the fan and hub with new ones.. but in the end opted to use a thermo-fan to suit my application.
oh, and make sure you check your hub, there should be 4 nuts that hold it in place. make sure they are all tight, it could be just the hub becoming loose