Dispute & question! It's a "free" web.
cases could directly affect noise in a few ways
You're right, they can & do. With a system that is not noise-reduced, they can make a significant difference. My point was a bit of a technicality in that case noise is the result of other devices; they don't by themselves make noise. Even though this seems like a trivial splitting of hiars, there is value in thinking of it this way.
In a system where the noise sources are reduced, there is little difference between a top quality case and the kind flimsy case used, for example, in my P4 quieting project
. Why? Because there is so little noise from components that case vibration and noise insulation are almost non-issues. Fan noise is the larger part of the noise in most systems, aside from the HDD. How much you can reduce fan noise by slowing them down (without causing overheating) depends partly on how well air flows through the case. Decouple mounting can make any case utterly free of vibration-caused noise. Thus my comment.
BTW, with the quietest systems, even with all the components just laid open on top of the test bench (drives and PSUs on foam), the noise level is low enough that visitors get shocked. What I look for is a case that holds it all together with minimal airflow impedance, some noise insulation and allows decoupled mounting of the HDD -- and
, I am starting to think, the PSU