Why Your Tankless Water Heater Is Making Noise — A Simple Homeowner Guide

A tankless water heater is designed to run quietly. So when it starts humming, rattling, vibrating, or rumbling, something inside the system has changed. The good news is that most noise issues come from scale buildup — and they’re usually easy to fix.

Here’s the straightforward breakdown I give homeowners during a service call.

Common Noises and What They Mean

1. Humming or Vibration

This usually happens when scale builds up inside the heat exchanger, forcing water through narrower channels.
The unit has to work harder, which creates vibration.

2. Rumbling or “Boiling” Sounds

This is a classic sign of heavy scale. Water hits hot spots inside the exchanger instead of flowing smoothly.

3. Loud Ignition or Burner Noise

If the burner has to fire harder than it should, it can sound louder — almost like a small whoosh or thump.

4. Fan Noise or Whirring

Sometimes debris buildup or restricted airflow makes the internal fan louder.

In almost every case, these noises are tied to restricted water flow or scale buildup.

How a Professional Flush Fixes Noise Issues

Here’s what happens during a proper descale:

• The descaling solution dissolves mineral buildup
• Water pathways widen back to normal size
• Burner no longer overheats
• Flow becomes smoother
• System stops vibrating from internal pressure points

Most homeowners tell me the noise is completely gone by the end of service.

When Noise Means Something Serious

Rare but possible:

• Vent blockage
• Failing fan motor
• Gas pressure issues
• Loose mounting hardware

During maintenance, these issues are spotted quickly and are referred to licensed repair professionals.

The Bottom Line

Your tankless water heater shouldn’t be noisy. If it is, a flush almost always fixes the problem — and keeps the system running quietly and efficiently.