Try larger fans running at a slower RPM
Thats exactly what the iMac G4 does - it has a fairly low speed fan running at the top of the dome, pulling air up through the base and around the inside metal casing. Heat pipes carry the heat from the cpu out to that case. They're quieter than the HD itself.
The convection iMacs have the G3 in contact with a rather massive plate of aluminum that's also connected to a mesh just on the inside of the case - most of the air that passes up through the machine goes through that mesh. Same kind of idea
- lots of air, moving slowly. instead of smaller patches of highspeed air.
