When seeking to lower the noise in your PC, we always suggest that quiet enthusiasts eliminate noise from the top three to five noise makers in their computer before doing anything else. In order to build the quietest computer possible, FIRST, make sure to make the following as quiet as possible:
If you have a noisy video card, make sure you've silenced that too by either purchasing a fanless video card to begin with or else by outfitting it with a quiet video cooler.
Once you have done these things, it is time to turn your attention to the fine details of building your Quiet PC. Case Insulation is one such important detail.
Computer case insulation won't eliminate all noise from an already noisy computer (although it would make it more bearable). Computer case insulation will, however, take that last bit of noise out of an already quiet computer setup.
We recommend Acoustipack case insulation for your quiet computer project. Acoustipack is relatively easy to install and include dense foam designed specifically to dampen the noise in your PC, safely and effectively.