Another standard issue that causes loud noises in tankless water heaters is a dirty flow sensor.
Tankless water heater noisy pipes.
Water vapor is a byproduct of combustion although in this case the vapor contains elements such as methane nitrogen and oxygen.
Or you can try to run down the leak yourself by checking the following.
A blocked or improper ventilation and even a defective or wrongfully set pressure valve can also make your tankless water heater loud.
Generally speaking there is a lower chance of hearing strange noises with a tankless water heater vs tank model.
Others reasons for a noisy tankless water heater model may be an issue with the burner the fan may be dirty or possibly a leak in the sealed combustion resulting in irregular gas combustion.
Although keep in mind that it is normal to hear water entering the tank when hot water is being drawn.
If your water heater is making noise like water is running you probably have a leak or a broken pipe.
Another common problem that can create noise in tankless water heaters is a dirty flow sensor which controls how much gas is sent to the unit.
Inside a condensing tankless water heater the combustion process superheats the water that enters your shower sink etc.
If the running water noise continues once the tank is filled you may want to call a professional plumber to investigate the situation for you.
If you hear a weird groaning sound from the pipes or the tankless hot water heater when you flush the toilet that could be the culprit.
In that case you need to have a plumber install a check valve in the water line.
The sensor controls the amount of gas sent to the unit.