For anyone in interested in building a silent PC, I have some simple advice: use a Noctua CPU heatsink and Noctua case fans. They are whisper-quiet, and I spent < $100 for all the fans in my system. They are the most premium-feeling computer product I have ever used; the CPU heatsink was very heavy and solid, and came with all the tools needed for installation including a screwdriver and thermal paste. Highly recommend, 10/10.
Adding to this: BeQuiet cases. You want that internal sound dampening on all sides of the case (so avoid tempered glass) to get rid of any annoying high-pitched coil whine, not just to silence the drone from fans. I have an overclocked 9700K with a BeQuiet case, the stock BeQuiet fans (which are quiet) plus a Noctua heatsink and extra Noctua case fan. After having adjusted the fan curves it runs cool and is inaudble (from less than 1m away) without headphones.
I'm not sure I can recommend it. I went from an Antec 900 to a BeQuiet PureBase 500 and although it's a good case for a good price, I'm disappointed in noise and thermals. Turns out, as many reviewers have pointed out and others long realized, you're better off with a more open case with bigger and slower fans than an enclosed one where you have to run your fans at higher rpms to get equivalent thermals. It's actually noisier and has worse thermals than my 13yr old Antec even after configuring for a quieter fan rpm profile. Although the newer DX model seems to have improved a bit in that area of increased airflow which pretty much defeats the noise savings from insulated panels. All in all, it's still quiet enough that it's not distracting, good enough for me.
