The AS18 unit I bought on EBay, with two airbrushes (see photo above) is a lot quieter than I was expecting after watching videos. It is a piston type which I understand is a lot less noisy than a diaphragm unit. Yes, my wife can hear it in the other room when I have the door and windows open, but does not disturb her TV viewing. I have seen the same unit in a local model shop costing a lot more - even without the two airbrushes and 1m of air pipe.
Turn it on and it pumps up to 100psi then cut out, holding that pressure even if turned off overnight. Start spraying and the pressure drops rapidly to what you have set (ie 20psi) which stays fairly constant provided you don't hold the air button down constantly. In practice I release the button briefly after each spray pass. No point in wasting paint. With practice I can get a reasonably constant 20psi when passing the brush across the model.
As stated before, I am brand new to airbrushing, and I am sure that as my experience with this grows I will gradually get better with what I want to achieve.
Bob K