Yeah, it certainly wasn't the quietest choice for form-factor, but the fact remains: all server grade hardware are not optimizing for noise. They are meant to be running in datacenters, not livingrooms, so noise was never a concern for them. A nice thing about consumer-grade hardware is they are optimizing for both sound and power consumption because those devices are designed to be around humans. So I certainly hope consumer-grade hardware survives.

I have a 4U with noctua fans and the loudest part of my rack is the harddisks

Not only the loudness, the small fans have subjectively more annoying sound even if they were the same volume. Much more shrill than a large fan.

Sure. But are there actual limits on how much noise they're allowed to make?

This is all built to be put in a place where noise is not an issue