The low humming sound is more than likely caused by the water pressure which can cause a variety of odd problems including these noises. High pressure can damage toilet valves and other devices connected to the plumbing. If you're on a city water system, ask your neighbors if they are having similar problems. If you have a well you probably have a water pressure meter. If you don't have one, borrow one or, buy one at the hardware store and check the pressure. If your pressure is above 60psi you may want to adjust your pressure reducer or install one. I understand some water systems have higher pressures in the main lines and pressure reducers between the main and your house, at the meter. In this case, if the pressure regulator is defective you could have high pressure even if your neighbors don't.
Sometimes pipe noises are caused by the pipes rubbing or bumping inside the walls because they aren't strapped down well or, because their stud pass-through holes are too small or off center. If this is the cause they've probably made the sounds since the house was built or, the wall was repaired.