Fan connectors can be classified as PWM (Pulse Width Modulation) Mode and
Voltage Mode. PWM Mode fan connectors provide constant 12V output and adjust fan
speed with speed control signal. Voltage Mode fan connectors control fan speed by
changing voltage. Therefore, when you plug a 3-pin (Non-PWM) fan to a PWM Mode
fan connector, the fan speed will be always maintained at 100%, and that could be
noisy.