I recently built a dual-Opteron workstation using a Tyan Tiger K8W\nmotherboard and a pair of retail box Opteron 246 CPUs (2GHz, 130nm), in\nan Antec case with a pair of 80mm exhaust fans and a dual-fan 400W power\nsupply. The system is acceptably quiet when I first turn it on, but in\njust a few minutes the CPU fans wind up from the 3500RPM or so that they\nstart at all the way up to about 6000RPM-\-making a deafening racket.\nBut the CPU temperatures read about 42C under moderate load; not hot\nenough to justify that high a fan speed, at least for not a workstation\n(it'd be just fine in a noisy server room).\n\nSo who/what is causing the fans to speed up that high? Does the\nmotherboard increase the voltage to the fans when the temperature goes\nup, or do the fans themselves contain temperature sensors? Is it\npossible to adjust the speed ramp? I wouldn't mind trading a few\ndegrees for some peace and quiet, as I'm not overclocking or anything\n(just developing multithreaded software, and I need multiple real CPUs\nto test it properly).