Now I need overflow help

[mark]

You pretty much have a Durso already and looks like setup okay as water level seems steady without the air-hole blocked. No sure how much difference replacing the one elbow to a Tee and cap (for a classic Durso) would make but all I know is when they are tuned, the drain pipe itself is quiet.

If the noise that causing the problem is the water fall effect into the overflow compartment, increase the height of the vertical pipe so the water level is higher in the overflow (less drop, less noise).

Difference on mine, between overflow compartment level and tank level is about an inch. As long as the overflow level is less than the tank (even 1/4") functionally it's correct.