The sensor controls the amount of gas sent to the unit.
Tankless water heater noisy pipes.
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.
If your water heater is making noise like water is running you probably have a leak or a broken pipe.
Good luck with your project.
Another standard issue that causes loud noises in tankless water heaters is a dirty flow sensor.
In that case you need to have a plumber install a check valve in the water line.
Inside a condensing tankless water heater the combustion process superheats the water that enters your shower sink etc.
Other sources of noise include blocked or inadequate ventilation or a defective or improperly set pressure valve.
Even though a toilet only uses cold water i have seen some toilets do this.
Or you can try to run down the leak yourself by checking the following.
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.
Although keep in mind that it is normal to hear water entering the tank when hot water is being drawn.
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.