I think Tal’s response is fine. People (including me) often ask about help with a solution they’ve already decided on, without explaining why they eliminated other alternatives that would solve their problem. Sometimes it’s good to back it up. Further, there are more than just OP reading the responses so it might apply to others now and in the future.

Hah! Actually I don’t know anything about construction, but have worked in a major rotational drive manufacturer’s research labs, albeit not on the mechanical side of things.

You’re right that it wasn’t what OP asked, but I’ve spent some time reducing PC noise because I was doing some audio work near one, and spent time trying to reduce PC noise. I’ve also rigged up automated spin-downs and some of the other topics mentioned here. Wanted to aim him at some resources that I’d hit, as I’d found that generally, trying to use low-noise components was more-constraining and less-effective than either moving or damping the source.

For one, I’ve spent quite a lot of time designing and building the enclosure from scratch (it’s a unique and original design from my creation).

Ah, gotcha. Depending upon the design and how much space it has, that might actually bring up another possibility. Some of the issue with drive noise comes up as it’s mechanically-transmitted to the case. If you haven’t already, two ways to reduce transmission of drive noise I’ve seen used is to either mount the drive on silicone or rubber gaskets or – and this requires more space on a per-drive basis – suspend it by elastic bands.

I don’t know if that’s something that would be trivial or a lot of work to add to your custom NAS, but I’ve done both and found that it was helpful with drive noise transmission to metal cases.