Tankless Water Heater Noise Problems: What’s Normal vs. What Signals a Plumbing Issue

Have you ever turned on the hot water and heard a sound from your tankless unit that made you stop for a second?

If you’ve recently noticed new sounds coming from your tankless unit, the first question usually isn’t “Is this broken?” It’s “Was it always doing that?”

That uncertainty is common. Tankless systems operate differently from traditional tank heaters, and the sounds they make can vary depending on water demand, gas flow, mineral buildup, and the cycling of internal components.

Some noises are normal. Others are early signs that your plumbing system or the unit itself needs attention.

Understanding the difference helps you avoid ignoring a real issue and avoid calling for water heater repair when nothing is actually wrong. This guide breaks down exactly that. 

Normal Sounds Tankless Water Heaters Make

Before assuming there’s a plumbing issue, it helps to know what standard operation sounds like.

1. A Low Fan or Humming Sound

Tankless units contain internal fans that assist with combustion and venting. When hot water is running, it’s normal to hear a low hum while the system is active.

If the sound stops when the hot water stops, that’s typically normal operation.

2. Clicking When Water Turns On

Most tankless systems use electronic ignition. When a hot water tap is opened, you may hear a brief clicking sound as the burners ignite.

A few clicks at startup are expected. Continuous clicking after ignition, however, is not.

3. A Soft Whoosh During Operation

When gas burners engage, a subtle “whoosh” sound can occur. This is simply the sound of combustion starting and stabilizing.

If the noise is steady and not accompanied by vibration or rattling, it’s usually nothing to worry about.

Noises That May Signal a Plumbing or System Issue

When sounds change in volume, frequency, or character, that’s when a closer look is warranted.

1. Loud Knocking or Banging

Knocking sounds are not normal for tankless systems.

This often indicates mineral buildup in the heat exchanger. As water flows over scaled surfaces, it can cause uneven heating and pressure fluctuations, leading to noise.

In areas with harder water, this buildup happens faster. When knocking develops, professional flushing or water heater repair may be needed to prevent long-term damage.

2. Persistent Rattling or Vibration

A rattling sound can indicate:

• Loose internal components
  
• Mounting hardware that has shifted
  
• Venting connections that need tightening
  

Because tankless systems are mounted to walls, vibration can travel through framing and amplify sound.

A plumber can check mounting stability and internal components to prevent further wear.

3. High-Pitched Whining

A whistling or whining sound often relates to restricted water flow.

This could be caused by:

• Sediment in inlet filters
  
• Scale buildup
  
• Partially closed valves
  
• Pressure irregularities in the plumbing system
  

When flow becomes restricted, the unit works harder to maintain temperature, which can produce abnormal noise.

4. Repeated Ignition Clicking

If the system continues clicking without fully igniting, that can indicate:

• Gas supply issues
  
• Ignition system failure
  
• Sensor problems
  

Repeated failed ignition attempts should not be ignored, as they can prevent reliable heating and may require professional water heater repair.

Why Tankless Systems Are Sensitive to Maintenance

Tankless water heaters have smaller internal pathways than traditional tank water heaters. That design makes them efficient, but it also means mineral buildup and sediment can affect them more quickly if regular maintenance is skipped.

Annual flushing helps remove scale and protect internal components.

When maintenance is neglected, noise is often one of the first signs that something is developing inside the unit.

When to Call a Plumber

You should consider calling a plumber if:

• The noise is new and noticeably louder
  
• The sound continues even when the water is not running
  
• The hot water supply becomes inconsistent
  
• Error codes appear on the unit
  
• Pressure changes accompany the noise
  

In many cases, early service prevents larger repairs later.

Addressing Noise Before It Becomes Damage

Not every tankless sound means something is wrong. But changes in sound are worth investigating, especially when they appear suddenly or worsen over time.

If your tankless water heater is making unfamiliar noises, Rockwater Plumbing can inspect the system, evaluate your plumbing setup, and determine whether simple maintenance or water heater repair is needed.

Scheduling service early helps protect the system and extend its lifespan.