the unit should have two fans, one inside, the evaporator fan and one outside the condenser fan.
the inside fan runs all the time, the outside fan runs when the compressor kicks on.
each can be replaced, usually noise indicates that it will fail in the near future, usually the bearings/bushings in the motor are becoming worn out.
both are easy enough to replace
often times the interior fan is a "skeleton fan" and the exterior one is referred to as a "bowling ball fan" or "cannonball fan" major appliance parts houses have both, take note of the rotational direction and the speed of the fan when ordering a replacement. Service the unit with it unplugged.
