We easily overlooked factor is "220V Cooling Fan" bearings. Because the fan speed rotation friction between the shaft and the bearing to the collision, it is also a major source of fan noise. Fan noise and friction, air flow related. Fan speed, the greater the amount of wind noise caused will be greater, additional fan vibration itself also can not be ignored. Of course, high-quality "220V Cooling Fan" of the shock itself will be small, but the first two were, but it is difficult to overcome. To solve this problem, we can try to use a larger size "220V Cooling Fan". Should be under the same amount of wind, the large fan noise at low speed work should be less than the work of a small fan noise at high speed.