Overflow box noise is generally from the water falling into the box. It can be avoided with a piece of thin plastic formed roughly to the box shape, inserted on an angle towards the inside leaving a small gap directing the flow back to the wall. Valves open by turning to the LEFT. The valve has to be closed enough so the water level stays above the inlet. It will take time to get it right. Make an adjustment & wait, if needed make another one. The closer you get to perfection the more time you wait between adjustments. I start with a wide open valve.