A friend of mine just assembled his new PC ( dual XEON , yes ! ) and wondered why the CPU fans did not adjust their speed according to temperature as told in the mainboard-features, because they where the loudest thing in the whole PC.
After fooling around with ASUS and the Q-Fan technology on their new mainborads he found out by making voltage readings and testing the CPU-fans attached to a regulated powersupply that the original Intel CPU fans are regulated themselfs !
They will do the same high rate of RPMs needed for propper cooling from 5 Volt to 12 Volt ! Therefore the regulation done by the ASUS board, by lowering the fan-voltage, did not work. bdw: This also explains the high price of original Intel CPU coolers.
Fix: Buy a cheap 12 V Fan that fits your CPU cooler and the regulation will work and your PC will be more silent.