I worked for a hard drive manufacturer, La Cie, back in the last century. They ran 12v fans on the 5v bus to spin them slower. I liked my 128K Mac because it had convection cooling.
It's hard to find 2nd-hand Mac Minis today. The current one is slower than the one it replaced, but the DSL connection is the choke point, anyway. Come July-September this comes to market, and I'm thinking it would be a nifty upgrade:
https://www.kickstarter.com/projects...e-raspberry-pi
It's a $35 Raspberry Pi with a $99 audio daughterboard. Fanless.