Water heater makes a constant hissing noise with power on or off?

You have a water leak under the house, but need to know if it's a gas heater or electric. check the Pop off valve on top of the tank it has a short handle and should go threw the floor pull up on the valve and let it sit back down, if it stops the high pressure pop off valve is leaking, you can buy it and replace it for 5 or 10 bucks, if that does not stop it look under the house. Also use a broom stick and put the round end at the wall and the other end near your ear and check the wall and floor for the noise the closer you get the louder the sound. Hope this helps. If you look under the house and see water you know you got a leak in the pipes or the tank itself.

Steam, sounds like steam, but most water heaters of the electric variety make some noises, but you may want to consider having a professional come out and evailuate it and make sure its not a serious problem, for safety, check here, ask the question, BUT also call someone out to check it out, because if there is a problem, it can cause a lot of damage.