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&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.

Terminology

• PRV (pressure reducing valve) used to reduce the pressure in a piping system to a set maximum.
• T&P (temperature and pressure relief valve) used on a piping system as a safety devise to reduce the built up pressure in a piping system.

Explanation

It is not uncommon for noise to be generated by the PRV on the incoming water supply line. This typically means that the valve needs to be serviced. Often it can be caused by dirt or grit that has got into the valve, or ware of the internal parts. Often you can take it apart and clean in. There are also service kits for most PRVs that you can buy to replace the internal components. However, due to the cost of labour, and the relative low cost of a small PRV, most companies will just replace the valve.

Most plumbing codes allow for a maximum pressure of 80 psi for domestic water systems. This pressure is to prevent damage or failure of the plumbing fixtures. It also reduces water waste and therefore cost. As such, the PRV should be set for the desired maximum pressure and not by when it stops making noise. A reasonable setting would be 60 psi.

There are other reasons that PRVs will make noise other then dirt or internal component failure. All PRVs are designed to be operated between a minimum and maximum flow rate. Using the wrong size PRV can also cause noise to be generated.

The pressure that you measured at the water heater may be isolated to that of the incoming water pressure. This is due to the fact that heating water in a semi-closed loop will result in expansion and increased pressure.

The pressure created by the water heater can be dealt with in a few ways. In older installations, check valves were not always installed on the cold line that feeds the water heater. This allowed for the pressure to absorbed back into the cold portion of the system and in some case back into the city water line. Most current codes (or even city bylaws) require that a check valve be installed and thus the expansion must be dealt with. Typically the use of an expansion tank is used or a thermal expansion valve.

Even when using these devises, expansion may still occur. And in some scenarios, such as a failure in the water heater’s controls, excessive expansion may occur to the point that the tank may rupture and explode. This is why all water heating tanks must be installed with a T&P valve. Typically they come set at 150 psi and 210°F for residential water heaters.

So even though you measured 120 psi at the water heater, that does not mean that the PRV is set to 120 psi. In addition, 120 pst measured at the water heater, especially if the measurement was taken just after the water heater was heating may be normal.

Recommendation

From what you described, and assuming that the plumbing company was competent (and there is no reason to assume that they are not), this are probably safe.

If you want to further reassurance, I would suggest for you to do the following:

1. Confirm the pressure that your PRV is set at and adjust if needed.
2. Confirm that you have some means of dealing with the thermal expansion caused by the water heater.
3. Confirm that your water heater has a functional T&P valve.