Loud Overflow

[Werbo]

I am finding it extremely challanging to get the overflow quiet. I have a 5X5 inch overflow with a 1 1/2 inch bulkhead that is connected to a 2 inch pipe that goes out the centre of the back wall of the tank through a 2 inch bulkhead. On the outside of the tank I have a T. One end of the T goes up above the water line and is capped with a small hole to let in air.
The other end of the T goes down through a 2 inch ball valve before going to the sump through 2 inch pvc pipe. My return pump is approximately 750gpm going through 1 inch spaflex to a simple u-shaped return to the tank.
If the ball valve is left wide open the overflow is constantly empty and sounds like a bathtub draining the last 1 inch of water before it is empty. It is very loud.
I have tried adjusting the ball valve to find a happy medium but it seems impossible as the tank will become quiet as the overflow fills but then the dispaly tank will slowly begin to overflow the top and flood the house. For days I have fiddled and feel it is only a matter of time before I flood my house.
Extremly frustrated. Any help would be appreciated.
ps. There is no room in the 5X5 overflow to fit a mini durso. Already tried that.