If there is low-frequency fluctuation in water pressure, plumbing will respond if there is air trapped anywhere within. If you tried everything else you can think of and can't pin it down, it might be worthwhile to shut off the water outside the house. Similarly with natural gas, if you have natural gas, which isn't liquified but is highly compressed and fluctuation in the gas pressure can cause those pipes to vibrate. Also shut off the hot water heater and leave it off for a day to allow it to reach room temperature, to see whether this does anything.
Whatever it is I hope you find it.
Whatever it is I hope you find it.