PCV or PRESSURE CONTROL VALVE
 

Our house has a very low hum inside the bathroom wall. After doing some online research the most likely explanation is the pressure control valve needs to be replaced. Has anyone else heard of this? Thanks,

Not likely to be a valve. A pressure control valve or pressure regulating valve would be located in the garage and it would be accessible. All plumbing valves in your house should accessible. To install any valve behind a wall would violate the plumbing code.

Do you know why a transformer hums?

It doesn't know the words.

Is the hum there all the time or just when water is running, or when the fan is running? I have seen old style CFL or LED lights that would hum, very rare but it did happen. I just purchased a leak detecting set of microphones, that would be able to listen inside the walls and to the pipes if they are accessible this would confirm or eliminate on source of the sound.
Can you provide more info?

You sure it’s not the exhaust fan? I had a customer who tried for three months to figure out the source of the noise. Turns out it was the exhaust fan in her master bathroom