There are various components that are dedicated to creating a PC that's as quiet as possible, here are some of the options.
One of the best ways to keep noise down in your system is to work preventatively. Although it's possible to locate and eliminate excess noise once you've got a PC you want, it's preferable to build a PC that has quietness in mind from day one. Everything from PSUs to hard drives are available with 'silent' variants that make less noise than a standard version, though they usually trade off something in terms of price or performance to do so. The upshot is that no silent PC will ever be the fastest you can get for your money - but if you're sensitive to the noise your PC makes, it'll definitely be the least irritating.
So, if you're looking to choose some quiet components for your next PC build, this guide has everything you need to make the right decision.