When no water is being used in the house, there often is a repetitive hammering sound in the plumbing which starts loud and then gradually fades away. Could this be a faulty water pressure regulator? Existing one is 23 years old.

Take your hand and feel where the vibration's the strongest. If it's strongest at the PRV then it's faulty and should be replaced. God luck, Tom

Not necessarily. Pressure regulator would make noise when water flows through it. If no water is active, pressure regulator just sits there...

However, Pressure regulator acts as amplifier of noise that can be coming anyplace from your house. Investigate. See what is running / active when you hear the noise again... Back to you...

The repetitive hammering sound in the plumbing occurrs when there is no usage - commonly early in the morning. I can stop the hammering by turning on a faucet, making me wonder if the pressure regulator is the problem.

It is more likely high pressure problem caused by faulty pressure regulator or heat expansion. Do this...

1. Measure pressure on the inside of your house. With all fixtures turned OFF and water heater cold supply valve in OFF position as well, observe pressure gauge for about 15 minutes. See if pressure slowly builds up. If positive than replace water pressure regulator.

If negative, then leave water off but OPEN ONLY water heater cold water supply valve. Observe gauge for next 15 minutes. See if pressure goes up. If positive, install Expansion tank.

Let me know what you've found out... Milo