I ran into a lot of videos a few years ago about "mineral oil" submerged computers. If you want to go that far and to that expense you might try an internet search on mineral oil cooled computers which brings up lots of variations.
I came to those videos by accident trying to look up how to silence my own normal PC. For a while I used a Sapphire Ultimate Radeon R7 250 which is fanless with huge fins. I've now got an Nvidia card which I made sure only used one expansion slot, as those using two allow a lot of extra noise.
I only got brave enough to leave my Sapphire behind because I did a lot of sound-proofing of my computer case. I discovered long ago the Antec 300 I bought for my first build happened to a have a lip on the side-covers that held standard poster board very well and that was my first step to muffle my PC. Then I got into modding and came up with my own idea for an "air scoop" inside the case here at my website
, which did quiet things a bit, together with more poster board.
Then about two years ago I felt I needed a larger case. I went with an Antec 302 which at first seemed like a mistake, but again I found a modification, as well as the same existing lip on the side-covers. This time I google learned about self-adhesive vinyl floor tile, tried that instead of the poster board, and also purchased Silverstone dampening foam. I absolutely packed the front panel with that adhesive foam around the air filter screen, closed the unused fan mounts and I'm very happy with the results. One drawback is that the tiles I added makes the PC heavy, but I never move it anyway.
Hope this all helps.