For the past 3-4 years, we have heard an intermittent
humming/vibration sound in our house. The sound is loudest in the
master bedroom (of course) and eminates from the wall on my side of
the bed. The sound is loud enough to hear in most parts of the house.
I have gone to great lengths to find out what is causing this sound.
This past weekend while replacing the auto-fill valve on our pool I
finally discovered how to make the sound stop.

In order to replace the valve I had to turn off the water supply. As
soon as I turned the valve, the humming stopped. After replacing the
auto-fill with a new one and turning the water supply, the humming
returned. Turning off the auto-fill every night so I can sleep is not
a solution.

There is a back flow preventer valve on the auto-fill line.
Considering the auto-fill valve itself is new, I no longer suspect it
as the cause of the humming. So now I suspect it is the back flow
preventer valve for lack of a better answer. Has anyone ever heard of
these valves making noise? What would be in their design that would
cause this?