40 HDD is a lot.

Check out 45 Drives. They have a homelab focused offering that has 15 bays https://45homelab.com/#product

To get 40 hard drives, you’d be looking at something like their Storinator S45 https://www.45drives.com/products/storinator-s45-configurations.php

Nice work! Always satisfying to see an idea reach the real world

Where are the external drives living? Hard drive make their own noise. Might consider going with solid state if you want silence.

One example of this that’s making the rounds: https://www.asustor.com/en/product/FS67