A metal case would be a priority for someone like me that does location recording though. A plastic case doesn't rackmount easily and doesn't hold up as well when hauled around. But that is still good to know for studio use. I do know that some metal cases are lots quieter than others and isolating hard drives and optical drives is the same process regardless. What most companies do not do however is line the inside of the case with damping material and line the drive bays in similar fashion. Quiet cases tend to be at least three space and much more usually four space because of the size of the fans. Large fans moving slow are much more quiet and move more air than smaller fans moving at even medium speeds.