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.
the noise is coming no the from the water going over the weir and air going down the pipecan 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.
Do you have a valve on the return side?Sadly I can’t slow down the return pump it is already on it slowest setting