I have that same overflow and mine is pretty quiet. First thing I see is the water level ,it is very high. Try adjusting the little straw vent on top , the holes on the side are more then a syphon break it helps with draining they kinda stream line the water flow down the pipe but the water level can’t be that high.
How much volumn do you have going backing of the tank? Too much will overload the overflow and raise the level in the overflow and the water level wll rise and fall and will be very loud.
Also the drain will settle down some once it has ran for a little while it will find a balance
I tried over and over again to get my durso to be silent. The drain was totally silent, but no matter what I did it was loud when it hit the water in the sump. I switched to a herbie and I will never go back.
I agree with bill. The water level seems too high. I have the same overflows and the water level is lower at the same level as the holes.
I have a quiet one 2200 pump. It’s rated for 455 gph at 4’, which is approximately the distance from the pump to the top of the return. I replaced the tubing from my return to the bulkhead with 1 inch ID. The flushing noise has gone away and the water level is consistent, but it’s at the same level as in my picture. Placing my finger over the straw will make the water level go down to just above the siphon break hole. I tried moving it up and down, but that doesn’t seem to make a difference. At this point everything seems to be working correctly and it’s fairly quiet. Should I worry that the water level is that high? It’s only an inch above the hole.