Inside be quiet!
There are many ways to turn a loud PC into a silent one - like using silence-optimized fans, SSDs instead of HDDs or decoupled mountings for case fans or HDD cages. An important part of a really quiet PC doesn't appear on too many radars as it's really inconspicuous and mostly taken for granted: the noise-dampening mat. But what does it do exactly?
In the early noughties be quiet! was one of the first producers of specialized noise-dampening mats. Back then our mats were quite colorful, by now we've resorted to anthracite which is more pleasing in pretty much every conceivable way. We have several noise-absorber kits in store, optimized for mid-size and full-size towers. A carefully selected set of noise-dampening mats should be part of every modern PC, just like an efficient power supply and quiet fans. They are no panacea, though - merely putting a mat into a loud PC does not magically turn it into a whispering miracle. It's part of a full silence concept, the cherry on top, which makes the difference between "it's fine" and "it's really silent". All of our cases are fully equipped with noise-dampening mats of course, with high-grade versions in the case of our silene-optimized ones.