I have a gas fired water heater, cylinder type, with the fan on the top. Every time I want to run my hot water there is a sound from the bottom of the water heater, something pops.
Is it dangerous? Do I have to fix water heater, can I do or I have to call a dealer or technician?
Gas burner inside the water heater,located at the bottom of the unit, is what makes the popping sound. It doesn't matter are you turning the water heater ON or OFF, it can still make this sound.
Reasons for this "problem" are due to the low gas pressure and there is nothing you can do about it, except to call your utility company and complain.
What you can do is clean the combustion chamber, where the main burner is... together with the main burner orifice... as it might be dirty. Also check where the dirt is coming from and eliminate the cause.
If your water heater is equipped with the air shutter reduce the air supply by adjusting it, as the excess air might cause the problem you are talking about.