Can I run Noctua 3-pin fans on 4-pin PWM connectors or 4-pin fans on 3-pin connectors?

Can I run 3-pin fans on 4-pin fan headers?

You can simply connect our fan's 3-pin connector to the 4-pin PWM sockets of today's motherboards so that the fourth pin that transmits the PWM signal is left blank (due to the shape of the connector, there's no risk of reversing 

polarity). Some motherboards are capable of controlling fans not only via PWM, but also by adjusting the supplied voltage. In some cases, this may require you to change BIOS options like "Fan Control Mode" from "PWM" to "Voltage" or "Analog".

Please consult your motherboard's manual in this regard. However, some motherboards use an "auto" setting by default that automatically chooses PWM or voltage-based regulation according to the connected fan. Automatically controlling the fan speed may not be possible on motherboards that pulse the power rather than lowering the fan voltage.