What mechanism controls the CPU fan speed?\n\nI recently built two nearly identical machines around the P4C800-E\nDeluxe motherboard. Both with P4 3.0GHz Northwood processors and\nusing the stock Intel heatsink & fan with thermal paste instead of the\nsupplied pad. The only real difference between the two machines is\nthe case and PSU. The first machine has a larger, better ventilated\n(and noisier) case, while the second is smaller, with just one 80mm\ncase fan.\n\nMotherboard monitor on machine A show motherboard temperatures\ntypically about 32C and CPU temps 33C at idle. In the second machine\nI have similar CPU temps, but the motherboard temp usually reads about\n37C. The CPU fan in machine A is at about 2700RPM at idle and may go\nup to 3200RPM under load. In machine B I have CPU fan speeds anywhere\nfrom 4000RPM to 4600RPM. Needless to say, the CPU fan noise from the\nsecond machine is louder and the fan drone is pretty annoying.\n\nSo, does the P4C800-E Deluxe use the motherboard temp sensor to\ncontrol the CPU fan speed? I would have thought it would only use CPU\ntemps to control this fan, which should give me similar fan speeds in\nthe two machines.