To get really low noise at low cost, if all other concerns were not critical, I'd just knock a hole in the wall and put the PC the other side really... too bad most of my connecting walls are to non-useable spaces (e.g. toilets, walkways or roof terrace).
Should be starting the build soon if my friend is up to it. Let's see how it sounds.. if not I can find alternative location to put the PC to get more distance from it.
Though my room is quiet, I think it should not be <10dbA anyway (definitely not during rush hour even though I have triple layered glass). 2-3 low noise fans should be able to achieve <15dbA and if I place it 4-6m away (as an option), it may be inaudible. My old Antec PC was about 23dbA with stock fans and that was abit noisy, but I think liveable with for some gaming performance.