There were no reductions in the drain, all 1.5 inch pipe. The centerline of the bulkhead is probably around 4 inches below the level of water in the tank.
In any case, it is academic now as I have solved the problem. I built a pseudo Herbie overflow with 2 1" drains coming off the 1.5 inch bulkhead using a T. It's angled so the majority of the flow goes down one side of the T and I have a ball valve on it to control the flow a la Herbie overflow. The higher end of the T is the emergency drain and dips around via 2 90 degree fittings. It is virtually silent now. Yay!!!!
I'll try to post some pics later.
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