This is normal. You said this occurs AFTER using the water heater. What you should have said is "after using all the hot water in the tank" I start to get this situation. IT CONDENSATION. Simple physics. Cold water flows into the NOW COLD tank because you have used all the hot water. Water condenses on the outside of the tank (inside). You then get the sizzle. If the water IS COLD ENOUGH when it enters the tank to reduce the tank's temp to the dew point temp condensation WILL form even if the gas burner IS ON. It stops because the water temp is above the DP temp and all condensation has evaporated. You might want to review the instructions that came with the heater. What I have explained is true even with ALL the components working properly. This could also occur if the gas burner was not on while the heater was being used and filling with water. In this case the water DOES NOT have to be so cold to overcome the heat from the gas burner. The only way I can think of to prevent this is 1. heating the inlet water which would be silly and 2. installing an electric water heater. Hope this helps.