Absolutely silent. My desktop at home makes more noise.

There are a few small quirks to having a immutable distro and only using flatpak. Your home is preserved though, so no issues there.

Most people probably wont even notice.

BUT: if you do find something that is not in flatpak, or doesn’t play nice, it is really easy to use KDE Kontainer and add a distro of your choice in a container. Anything you put here is never touched by SteamOS updates, interacts fully with your own user, and icons can simply be added to the menu to launch without having to touch or manually launch the container again. There are several guides and it is fairly painless.

Again, you may never need to. I just am adding that info here if anyone finds it useful.