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I made a switch with an ON-ON switch, where one position was full speed and another had a 10 ohm resistor in series with the fan. Be sure to use a high-watt rated resistor because a lot of current flows to those fans because they are such high power. The resistor made the fan run at about half speed and it was much nicer to listen to, and still had good airflow. You could also use a potentiometer, but I don't know how exactly because those have too much resistance on average.