Help! We have a humming noise in our pipes, sounds like a fog horn. Searched the internet and came across this.
"Shut off main valve, turn on water inside house, flush toilets and then slowly turn back on main valve."

That didn't work. There is also something my husband called a relief valve the builder put in but he can't remember what the builder said it was for. He tried to open it fully and close it but that doesn't seem to make a difference.

Any suggestions would be appreciated.