The way I slow down the fan is to: connect the RED fan wire to +12V and the BLACK fan wire to +5V, using a spare connector as a source of the voltages. This will run the fan at +7V at a slow, quiet, RPM. Test the fan by stoping it with your finger and letting it start up again. Do this about 10 times to be sure that the fan will always start with 7V. I've done this on my machines for many years now and have not had any problem from it. The temp will rise a little, but if the heatsink is large, it still keeps things cool.