Intermittently, I hear a low, vibrating noise after shutting off the water. It first occured after the shower was shut off. Now it happens after the washer shuts off. Yesterday I heard it happen all of its own for a few seconds. If I turn on a faucet the vibrations will stop, then return after I shut off the faucet. After a couple of minutes it will stop completely. When it is happening, I can feel a slight vibration in the faucet when I touch it. I feel it from any faucet – upstairs or down. Note that this will happen for days, then go away completely for a few weeks and then come back. Any idea what is happening?