A tankless heater is not silent. You may hear a fan, a click at start-up, or water moving through the unit. The useful question is whether the sound is new, much louder, or accompanied by a change in hot-water performance.
Notice the pattern
A rattle may be related to a loose cover or nearby pipe. A high-pitched sound, repeated banging, or a new vibration deserves a closer look. Note whether it happens only when a tap opens, while the heater runs, or after it shuts down. An error code or interrupted hot water is also worth recording.
Noise alone cannot identify a failed part. Avoid opening a gas appliance or working on its venting to investigate it yourself.
Share the details
Take a short recording if the sound comes and goes. Tell the team the make and model, any displayed error, and whether the hot water has changed. Our tankless water-heater service can assess the unit and explain the next step.
Separate normal operation from a change
Compare the sound with what the unit normally does at the same stage of a hot-water call. Does it occur only at start-up, throughout a shower, or after the tap closes? A fan or brief click may be part of normal operation for that model. A new sound together with an error display, water leak or repeated loss of hot water deserves an assessment.
If the unit is mounted near a bedroom, even normal operating noise may be bothersome. Tell the installer about that before selecting a replacement and ask about the proposed mounting location. If the current complaint is intermittent temperature, read what to record when a tankless heater turns cold. Do not remove the cover to locate a noise.