A mysterious knocking pipe can sound like a loose support or a failing valve. But for one homeowner, a noisy second-floor line only rattled when a Navien tankless water heater fired up — and one simple test pointed suspicion elsewhere.

That matters for household budgets, too: Households that switch to a heat pump water heater can save up to about $550 a year on energy bills, making careful diagnosis worthwhile before sinking more money into the wrong fix.

Here's what to know

The homeowner said in a Reddit post that every time someone used hot water anywhere in the house, a hidden pipe near a second-floor shower started making a rapid knocking or chattering sound. The home uses a Navien NPE-240A2 gas tankless water heater.

According to the post, the noise did not start happening right after the heater was installed. The Navien had run quietly for about six months, and the same second-floor plumbing had been noise-free for years with a standard tank heater. The homeowner also reported that the sound vanished if the unit was turned off and often stopped when hot-water flow was reduced.

One explanation centered on fuel supply rather than the pipe itself, with a user saying a 199,000-BTU tankless model can start restricting water flow if it is short on gas.

In the discussion, another user said: "Likely not enough pipe support."

A set-point test pushed the conversation further toward the gas-line idea. After the homeowner dropped the water temperature from 132 degrees to 100 degrees, the chattering went away, which seemed less consistent with ordinary pipe expansion.

More background

A pipe that stayed quiet with a tank heater may behave differently when a high-output tankless unit modulates heat and flow in real time.

The immediate timing of the sound was a big clue in the discussion. 

The homeowner added: "The noise starts the exact millisecond a hot water faucet is opened anywhere in the house. There is no delay." 

Explaining why the lower-temperature test could matter, one user said: "By lowering the temperature the burner won't need to be at maximum so it won't reduce the flow (should be able to tell if you open a tap and few seconds later pressure drop at the tap.)"

Opening drywall, replacing valves, or chasing what looks like a plumbing issue can drive up costs, even though the real problem could be tied to fuel delivery, pressure changes, or vibration traveling through less-supported piping elsewhere in the system.

What can be done?

Start by checking whether the sound changes with water temperature, flow rate, or multiple fixtures running at once. A licensed professional can then verify gas-line length, pipe diameter, connector sizing, water pressure, and pipe support before anyone starts opening walls.

For homeowners already weighing repair versus replacement, a heat pump water heater can be a strong money-saving alternative to many conventional systems. In many homes, savings can add up to hundreds of dollars per year, especially when paired with rebates or favorable electric rates.

Cala makes customizable smart heat pump water heaters that help homeowners decrease their energy bills by heating water when it's needed. Cala's system matches hot-water production to household demand, reducing wasted energy while keeping showers and faucets ready when people need them. For households looking past a one-off plumbing mystery and toward lower operating costs, Cala shows how a newer water heater can give homeowners more control over both comfort and bills.

After the lower-temperature experiment worked, the thread shifted to confirming the suspected cause and deciding on the next call. 

Where can I learn more?

If you're dealing with a tankless water heater problem or trying to decide whether one still makes sense for your home, these stories dig into similar questions about performance, installation, and cost. They cover weak shower flow, confusing plumber advice, water-heater safety scares, and the tradeoffs and savings that come with tankless systems.

• One homeowner found showers go weak after a tankless switch whenever toilets flushed.

• A homeowner said cagey plumbers' advice left the whole tankless installation plan in doubt.

• Terrifying footage showed house-rattling explosions after a broken water heater started violently misfiring.

• For budget-focused households, how much money tankless heaters save depends on demand and rates.

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