PRV valves fail often- could be yours has too. But first things first:
Have you MEASURED the water pressure in the house? If it's too low it could be an issue with a tankless too, showing up as some sizzle & bang on the water side of the heat exchanger, as larger bubbles form & collapse on the hot walls of the tubing. This is more noisy than dangerous, but cuts into efficiency since the bubbles insulate, reducing the effectiveness of the heat exchange. IIRC some manufacturers spec 15psi, as a minimum operating pressure, but 25-35psi would be better if you can, since at higher flow rates there's a significant pressure drop across the tankless.