Welcome to R2R!
If the overflow is behaving, it is super quiet. Right now if I stand with my ear to the top of the tank the loudest thing is the Kessils. In the dead of night, you would likely hear the skimmer/return pumps/etc more. I could hear Gyres across the room as they ramped up and down for example.
At times the overflow needs to be fiddled with. If you find yourself doing that constantly you can replace it with a gate valve. Otherwise it will make noise from time to time (read 4AM :p). Its actually something i strongly recommend anyways.
Edit: Wow I made little sense lol