Your hot water heater is not a cat — so, if you can hear it making hissing noises, you can assume something is wrong. Some of the issues that cause hissing are rather simple to solve on your own, while others will require a call to your local plumber. Here's a closer look.
Loose T&P Valve
The T&P valve, or temperature and pressure valve, is a valve at the top of your hot water heater that's designed to let air escape from the unit if the pressure climbs too high. If this valve is a little bit too loose, it may make a hissing sound when pressure is released from the tank. There's really no reason to be concerned if you can pinpoint the hissing as coming from the T&P valve. You don't have to do anything about it — but if the hissing is bothering you, then you can try slightly tightening the valve with a pair of pliers. Do not tighten it all of the way, as some air must be able to escape to keep your tank from exploding.
Open Drain Valve
The drain valve is the valve at the bottom of the tank. It's intended to allow you to drain the tank and remove sediment during your annual maintenance session. If it was left slightly open or if it has started to crack or separate from the tank, it may make a hissing noise. Usually, the noise is accompanied by at least a slow leak. You may see some drops of water on the ground near the tank.
Try closing the valve (assuming it's slightly open) to see if this takes care of the sound. If not, you may need to have a plumber come replace the drain valve.
A Crack Near the Top
If you get a crack near the top of the tank, more air than water may escape from the crack. This sometimes makes a hissing noise. Look over the tank closely, and see if you can spot any cracks. You may notice a few beads of water that have leaked through the crack. If you do spot a crack in the tank, turn off the water supply to your hot water heater, stop using your water, and call your plumber immediately. A small crack can quickly turn into a big crack, which could lead to a flooded basement and lasting water damage.