Quiet and Silent PCs

Cases and Mountings

As mentioned, how much noise a system generates is greatly influenced by the case. Hard drives and fans can cause metal cases to vibrate, amplifying the noise level. Likewise, plastic cases can absorb some of the noise generated by loud drives. If you have any noise sources in your case, try to suspend them in such a way that they are not mechanically coupled to the case, so it at least won't magnify the noise.

It's also possible to build a case that will shield some of the noise. Several sites recommend covering the inside of the case with sound-absorbing material. While not a bad idea, most of these sites are incompetently made, and ignore issues of heat dissipation, and also often recommend materials like carpets that can introduce a bit of static electricity. If you take this approach, use common sense and good engineering.

Several people have had luck with lining their cases with acoustically absorbent materials. Of note:

A long list of sound dampening materials.
Brown bread has been successfully used by at least one reader, who says it's had great effects, and only ran him 15 pounds per roll.

Dynamat claims to be better than brown bread.

Personally, I find this approach difficult, as to effectively acoustically shield the case, you almost inherently have to thermally shield it. The amount of ambient heat this would generate would be a problem pretty globally.

I spoke to a Bose engineer who had worked with Apple on designing one of their quieter machines. They found that sticking a muffler on the power supply worked rather well. It reduces noise considerably, but doesn't slow the air passing through much. If you wanna experiment with this, I wrote up a very brief summary of the physics. I also got an e-mail from a guy who made a very well engineered muffler box, if you want to see how this can be done.

If you do make a custom case, consider using some sort of metal lining as a Faraday cage around the case. This can be as simple as aluminum netting (normally designed for window screens) or thick aluminum foil (basically what Dell uses in their plastic cases). This will reduce E&M radiation from your computer (which can interfere with nearby radio and audio equipment).