Silent fans are nice, but you still have too much noise coming from cdrom's/writers/harddisks etc...
I solved it almost completely by
locking the pc's in a closed cabinet.
The cabinet is against a cold wall of my house, so only problem with warming up occur during summertime.
But then it is too hot anyway in my place (topfloor of the house) to stay there long.
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Keyboards/Sound Units: Kurzweil 2600S, Roland VR-760, Acces Virus C, Roland G-800, Akai AX60, Minimoog, Machine Drum, Roland R8-M, mediastation x-76