A PRV creates a closed system. The check valve the water company will create a closed system - reinforcing the one you already have. ALL closed systems need an expansion tank. So, add one! As you use hot water, it gets replaced with denser cold water. When you stop using, the WH heats that cold, dense water, creating hot, lighter, expanded water. Your pipes don't generally expand to accommodate it...something has to give. It's probably the weakest valve in the house, but it could easily be a supply hose or the check valve in the PRV (it might have a high-pressure bypass) or more commonly, the T&P valve on the WH. Regardless, if the water pressure gauge you bought has a tattle tale hand, leave it on for say 24-hours...my guess is that the water pressure will spike at least once above the 50psi setting because of water expansion...stop that, your noise will probably go away. Where I live, all houses are required to have an expansion tank, regardless of whether it is a closed system or not to allow for the addition of a check valve...check valves prevent potentially polluted water from your house from getting back into the supply system and are a safety device many places are adding, or already have.