A plumber recently installed a new pressure reduction valve and expansion tank in my house (which previously had neither) because the water pressure was unusually high. About a week after the install, I started hearing a low, loud foghorn noise whenever I turned on any faucet in the house or flushed the toilet. The noise comes a few seconds after the water comes on, and it typically only happens after periods of no water usage, such as in the morning or when I get home from work. I spoke to the plumber who is coming back next week, and he doesn't think the problem is the PVR unit itself, but everything I'm seeing online indicates that usually is the problem. What could be causing this noise?
Update: the plumber came out and agreed it sounds like the PRV. He replaced it, and two days later I haven't heard the noise once.