[jyl]

A lot of PC cases don't use natural convection, i.e. bringing in cool air from the bottom and exhausting hot air from the top. Maybe check and change the location of the fans.

There are utilities that idle the CPUs when they are not doing useful work, which is most of the time.

Some motherboards can control the speed of the fans based on temperature readings at various locations on the mobo.

Many new desktop PCs use ducts to concentrate airflow over the hottest components. You can make some from cardboard.

Some of the noise we hear is vibration, which can be reduced by tightening loose things and using pads. Hard drives do this a lot.

How about posting pics of some of these homebrew machines and cases? I'd like to see them.