Not silent or quiet, but muffled. (That's probably the best overclocking fanatics can hope for!) Seriously, "Muffled Computing of San Jose, CA is announcing the release of their new fan mufflers and foam kits designed specifically to create a high performance computing experience free of excess noise." They look like well-built little add-on metal boxes, lined with damping, to act as vent tunnels or mufflers for fain exhaust. 5-15 dBA noise reduction with less than 1% increase in temp is claimed. Check out Muffled Computing.