I left home for 1 month vacation. Before I left, I shut off the water main valve. When I returned, I turn on the water main valve, since then, it frequently have the humming sound which last for about a minute or two. Sounds off if I immediately turn on any one faucet to let water out. But as soon as I shut the faucet off, sound comes in again. I also tried to shut off the main water valve again, and let all water drained out by turning on the basement faucet. But this won't solve the problem. Anyone have experience this and how to get rid of the bumming sound? Thank you.