The S5 stock fan probably blows at 250CFM, and is loud. If you put your hand behind the miner you can feel the push of the air really well from that one fan. If the ambient temps are 60-75F I would imagine the fan runs at 3000-3500 rpms and  keeps the temps 48c/49c which is exceptionally good. If I can find a 4 pin fan underclock/undervolter I would give it a try. The stock fan is working way too hard when it doesn't need to. About the noise. If the S5 is in line of sight to your eyes you will definitely hear it and it may bother you after a while. But if it is in another room that is across another room with a turn where you can't physically see the miner then it sounds completely different. Much more tolerable on the ears. Either way I'm replacing all my stock fans with the Noctua NF-F12 Industrial 3000 model. One fan one each end in a push pull config. Take about 20 dB away, but is not silent but MUCH more tolerable. And if in a basement you may not even hear it.

P.S. do not insert filters or anything like that. That will restrict a lot of air flow on any fan. Just don't be lazy and keep your rigs clean. When they need a cleaning, I take them outside and use my air compressor to blow them down and dust them off. Shiny like new.