Place the exit just above the water line. Is the water going into a filter sock? I would place the exit to the drain above the water line and drain into a filter sock. It will still make some noise but not as much. Yoy could also turn down the flow on the return pump via ball valve to reduce the flow through the sump. This will qiest it alittle also. This will also allow for much better skimming because of more contact time with water in the sump.
There will be noise, just part of the hobby.
Now, since you only have one overflow box I would also reccomend a Herbie Overflow design. It is a 100% quiet design. I installed it on my tank but I have two overflows and found it impossible to dial in 100% and it made me very nervouse. However I believe its really easy on 1 overflow.
That will require you to replum the whole system though basically top to bottom.