Hot water heater making a humming noise

My hot water heater is making a humming noise. It started when the park lost water for a day and we had a lot of air in the lines. I have plenty of hot water and when I turn it off, it is on electric only, it still makes the noise. so I don't think is an electric problem. Any ideas?
I should have said it only hums when I turn on the hot water faucet, any of them at it hums.

A Hummingbird must have flown in through "empty" water lines in the park. This has been know to happen from time to time. That is probably the reason the park lost water. Sorry I couldn't resist.

Have you checked the pressure relief valve to make sure it's not that? Can you hear the hum outside or just inside?

I really like hummingbirds!! But I haven't heard one like this:))I should have said it only hums when I turn on the hot water faucet. The sound can be heard in or out and I did release the pressure valve and it still does it.

Sounds like you have air trapped in there somewhere, when you checked the pressure release valve, did water come out, or a combo of air/water?

Just be careful and dont get burned from the hot water.....

I think it is the check valve used in the bypass circuit. Mine has been doing it for about four years now, works OK so I don't worry about it.

Some times a washer on faucet comes loose and will cause it to vibrate with water flow.