There is some maintenance needed on the unit. Follow the manufacturer's instructions to 'flush' the holding tank of any build up of sediment or minerals at the tank bottom or formation on the heating elements. The age nor the type (electric or gas) of the heater was stated. Is the hissing being released at the pressure relief valve on the tank's side near to the top of the unit? It needs someone to inspect it's operation for safety. Have you recently turned up the thermostat setting near the bottom of the unit? Try turning the temperature down, check for the minimum temperature should you have a dishwasher.
