Wind noise is the sound caused by blade cutting air. The higher the fan speed, the greater the airflow, the greater the noise.
Another factor is the bearing of the fan. It is also the main source of fan noise, because of the friction and collision between the shaft and bearing when a fan rotates at high speed.
Besides, the vibration of the fan itself cannot be ignored. Of course, the vibration of a high-quality fan will be very few. But the first two are hard to overcome.
Place a fan in an anechoic room with a background noise of less than 17 dB, 1 meter away from the Sound Level Meter and about 1 meter above the ground to test the sound level in the suction surface of the fan.