Is has become evident that the tuned length of the pipe is at a critical resonant frequency. The noise occurs in the evening, after the water heater has had time to heat the fresh charge of potable water and separate air from the water. This creates an air pocket in high places before the valves to a variety of outlets.
Part of the resonance is due to the pressure creating air pockets of exact length. when the valve is opened, water whistling past the riser to the outlet at excessive velocity creates sound similar to the highest tone of a water organ. It is necessary to rearrange the tuned length so air does not gather in the chambers and it is also imperative to reduce the water pressure to change the resonant frequency to a deeper harmonic.