Distribution system (supply air + exhaust air) in the upper floor floor

And from me also again: It depends on which level the system is running...

A difficult decision. On the ground floor we have 17.5 cm aerated concrete walls and a simple door in front of it. How low would the step height have to be to be noise-free in the hallway. Very hard to estimate. How are we supposed to assess this in advance?

How low would the running stage have to be to be noise-free in the hallway.

Resonance frequencies do not have to be disturbing only at high levels.

So we can only decide by chance and hope that the volume is right?

That's how it looks... even the perception of running noises is subjective...

You have been given two levers. Avoid noise by designing the system and ensure that the noise does not leave the room. If this topic is important to you, you can now take appropriate measures. Especially since these do not cost a fortune. I do not see the risk of overdoing it.