I finally have only one silent 120mm fan in PSUand another 200mm spectre pro fan for gpu/cpuSo absolutely the loudest noisemakers are HDDs. This prototype is for them. There's 80mm fan at 3V inside yellow foam, which blows upwards and keeps hdd's temps under 40C.first results are quite positive! 3mm "hardboard" can silence very well. Even cardboard was sufficient.