The case has an opening for a vertical fan but it is not needed unless you do heavy gaming and run a high end GPU. For video rendering and other tasks the fins are rated to keep it at a decent temp.

I have good hearing, and a HDD spinning or even a "silent" fan is still audible droning noise to me.

This build is totally silent. The PSU is over-rated on purpose because it has a below 30% max draw mode that is fanless.
So this case makes 0 noise.