"The purpose of this guide is to illustrate how-to convert a high quality, but rather noisy power supply (PSU) into a silent performer. There are many good power supplies available to the computer enthusiast. Unfortunately, a lot of them are noisy. The noise produced comes from the internal fan used to move air through the PSU and cool its internal components. Some new power supplies contain variable speed fans that increase in speed (and noise) as the current load and internal temperatures go up."
FULL STORY @ SYSTEMCOOLING (http://www.systemcooling.com/modules.php?name=News&file=article&sid=434)