Popping or rumbling from a Noritz tankless unit is most often scale buildup on the heat exchanger or trapped air in the line.
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Quick answer
Popping, rumbling, or a low humming sound from a Noritz tankless water heater is most commonly caused by mineral scale building up on the heat exchanger, trapped air in the water line after maintenance or a shutoff, or a fan or pump beginning to wear. A homeowner can safely note when the noise happens and check for recent plumbing work; heat exchanger and gas component work should go to a licensed gas fitter.
Need a licensed gas fitter for your tankless water heater? Call CANRO at (604) 359-1081 — residential natural-gas heating and hot water across Metro Vancouver since 1999.
A storage tank water heater rarely makes dramatic sounds because the water sits mostly still and heats slowly. A tankless unit like a Noritz works the opposite way: water rushes at speed through a compact heat exchanger while a burner fires hard for the duration of the draw, and any airflow, scale, or vibration issue shows up immediately as sound rather than being masked by a large volume of standing water.
Metro Vancouver's water is comparatively soft coming off the regional reservoirs, which is kinder to heat exchangers than the harder well water many Fraser Valley households deal with. Even so, scale still accumulates over years of use, especially in homes with older galvanized plumbing upstream of the unit or where the water heater runs hot most of the time.
Note whether the noise happens right at ignition, throughout the draw, or only after the unit has been idle for a while. If the sound started right after any plumbing work, a water main shutoff, or a filter change, trapped air is a reasonable first guess and sometimes clears on its own after a few draws. If the noise has developed gradually over a year or more of normal use, scale buildup on the heat exchanger is the more likely explanation, particularly if the unit has never had a descaling flush.
Beyond checking the inlet screen for visible debris if your model has an easily accessible one, opening the cabinet, adjusting gas components, or attempting a flush yourself is not something we recommend without training, since it involves isolation valves and, on some installs, chemical descaling solution handled incorrectly can cause damage.
| Water Condition | Recommended Descaling Interval |
|---|---|
| Metro Vancouver reservoir water, moderate use | Roughly every couple of years, verify against your model's manual |
| Fraser Valley well water or harder supply | More frequent flushing, since scale accumulates faster |
| High-demand household or elevated set temperature | Shorter interval than the manufacturer's baseline recommendation |
A proper descaling flush circulates a cleaning solution through the heat exchanger using isolation valves installed for this purpose. If your Noritz unit was installed without isolation valves, that is worth correcting, since it makes routine maintenance far easier and less costly going forward.
A loud bang at ignition, a burning smell alongside the noise, or soot near the vent termination are not routine maintenance issues and warrant immediate attention rather than a scheduled descaling appointment. Persistent rumbling that a flush does not resolve can indicate heat exchanger damage from years of scale, which on an older unit may tip the repair-versus-replace decision toward replacement, especially once the unit is a decade or more into service.
Safety note
If you smell gas, notice soot around the unit's vent termination, hear a rattling or banging you have not heard before combined with a burning smell, or your carbon monoxide alarm sounds, leave the home immediately and call FortisBC at 1-800-663-9911 from outside before doing anything else. Gas connections, the burner assembly, the heat exchanger, ignition components, and venting on a tankless water heater must only be serviced by a Technical Safety BC licensed gas fitter. CANRO holds gas licence LGA0043870.
Routine scale-related popping is a maintenance issue rather than an emergency, but a loud bang, burning smell, or soot near the vent means you should stop using the unit and call a licensed technician right away.
If isolation valves are installed, some homeowners do attempt a basic flush, but doing it incorrectly can damage the heat exchanger, so we generally recommend having a licensed technician handle it, especially the first time.
No. Our reservoir water has less scale-forming mineral content than many well water supplies, which slows buildup, but it does not eliminate the need for periodic descaling over the unit's life.
Trapped air introduced during a shutoff or repair is a common and usually harmless cause of temporary noise, and it often clears after the unit has run through a few normal draws.
Yes, a worn or cavitating recirculation pump can produce a rumbling or humming sound distinct from the unit's own heat exchanger noise, and a technician can isolate which component is responsible.
If the unit is older, has never been maintained, and a flush does not resolve the noise, a technician can assess whether heat exchanger wear makes replacement the more sensible long-term choice.
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