Water hammer, PRV, and safe water pressure at water heater
PRVs (or a Temperature and Pressure Relief Valve, as it should) have pretty much nothing to do with water hammer or the whining.

Whining

If replacing the T&amp;P valve fixed it, then good for you, but if not, check out this wonderful This Old House video.

Water Hammer

When valves close too quickly, the inertia of the water in the pipes causes a buildup in the force against the valve, causing a hammering sound to propagate through the pipe. You can simply install water hammer arrestors at the problematic appliances.

Water Pressure

120 psi is at the high end of what (GE) appliances are likely to tolerate. Without knowing too much more about the plumbing, all I can suggest is a regulator to manage the pressure.