I have a 1941 ranch house with two floors (SFH turned into a non-conforming duplex) and the pipes are making a loud banging sound every day and night. (Day is less followed, less heard because I'm not around and not focused on it, and usually working with the radio.) When I do make note is at night. It happens 3-4 times a night. At all ends of the house (so sometimes it's loud, sometimes softer depending on how far from my bedroom).
I tried turning off the irrigation and that did nothing. I changed the flapper on both toilets. Nothing. I turned off the water main, and that made it all go away. Of course I can't do this every night (Although I wish I could). I don't have ice makers or dishwashers in either unit. Essentially nothing is happening from the human side (showers, toilet, etc.) but I still have this nightly noise.
My neighbors don't have irrigation (I asked). I do live near a water district pump station but they tell me it's not that. One time the noises stopped when the downstairs tenant was away for a week (but not completely).
I'm desperate for ideas or ways to solve this nightly disruption. Help!