When you say leaking from the top....where exactly? Post us a pic. Is it where the water supply lines enter the tank? It may be time to replace it...how old is the unit? The way they make them now....most need to be replaced in about 8 years
I heard the hissing sound from mine last month. The tank was leaking and water was going down the flame tube in the center.
Time for me to replace it!
The hissing sound can be the water dripping down on the hot surface plate that the flame is located at.
The leak can be caused by a few things. If the water is at the top which is rare for where a hot water heater would leak can be the result of a faulty T&P valve if its located on the top of the heater. Most are located on the side these days however.
Another reason for the leak could be the earthquake fastening used. Some fasten the strapping to the top of the heater instead of around it. If the screws they used were to long they could have punctured the tank or perhaps just pushed in it enough to cause wear to a point in which it began to leak.
Still another reason for the leak is the fittings have failed where they connect. Although doubtful this is the cause, it does happen.
The best thing to do is call a plumber and turn off the water supply. There should be one valve on the top to stop the heater from leaking. It may take a while for the water to run out of the pipes, but its better then a flood while you wait.
Be sure to dry up the water as fast as you can. Do not want mold to develop. Last thing, check with home owners insurance if the leak was caused by the failure of a device or incorrect install with earth quake straps. You may find you may be covered. They will not cover a heater that just began to leak however if its beyond its warranty period. But it does not hurt to ask.