The gate valve is open all the wayThere should hardly be any water going down the red pipe; is the gate valve on your durso open all of the way? I have the same drain setup, and if I didn't have two gyres pushing waves into my overflow box, my plumbing would be silent.
can you tell where exactly is the noise coming from?
i'm in the process of building a system now and i had an annoying noise coming from the water moving throw the hole between the tank and the external overflow box.
i closed the gate valve a little bit to raise the water level in the overflow box to be about 1" above the the hole and its super quiet now
Sadly I can’t slow down the return pump it is already on it slowest settingPerhaps try slowing your return pump. I agree with @Funlad3, isn’t the red pipe the emergency backup, for if or when the main drain is overwhelmed or clogged.
Sadly I can’t slow down the return pump it is already on it slowest setting