Popping, rumbling, knocking — what's actually happening inside your tank.
Your water heater started making sounds it didn't used to make. Popping. Rumbling. Maybe knocking or crackling. It's unsettling — especially when you don't know what's causing it.
The most common cause? Sediment buildup at the bottom of the tank. But not all noises mean the same thing, and not all require the same response.
This is the most common water heater noise, and it's almost always sediment-related.
What's happening: Minerals from your water supply settle at the bottom of the tank over time. When the burner heats the tank, water trapped under this sediment layer boils and creates steam bubbles. As those bubbles escape through the sediment, you hear popping or crackling sounds.
Is it dangerous? Not immediately. The tank isn't cracking. Nothing is about to explode. But it does mean sediment has built up significantly — and that affects efficiency and lifespan.
What to do: This is a signal to address sediment. See our guide on whether sediment is dangerous to understand the timeline.
A low, continuous rumble — like distant thunder or a boiling pot — is another sediment indicator.
What's happening: Same mechanism as popping, but with more sediment. The rumbling is steam and water churning through a thicker layer of mineral deposits.
Is it dangerous? Not in the short term, but rumbling usually indicates heavier buildup than popping. The heater is working harder, using more energy, and wearing out faster.
What to do: Consider flushing the tank or having a professional assess it. The longer you wait, the harder the sediment is to remove.
Sharp knocking or banging is different from popping — and may not be sediment at all.
Possible causes:
Is it dangerous? Water hammer can stress pipes over time. Thermal expansion is usually harmless. A loose element should be tightened but isn't an emergency.
What to do: If knocking happens when you're not using hot water, it's likely thermal expansion (normal). If it happens when appliances run, it may be water hammer (consider a water hammer arrestor). If you're unsure, a plumber can diagnose it quickly.
A hissing sound, especially from an electric water heater, can indicate a different issue.
Possible causes:
Is it dangerous? Sediment on elements reduces efficiency but isn't dangerous. A leak needs attention. Condensation is normal during heavy use.
What to do: Check for visible leaks around the tank. If you see water pooling, that's a separate issue. If no leak, sediment is the likely cause.
High-pitched sounds are less common but worth understanding.
Possible causes:
Is it dangerous? Restricted flow is annoying but not dangerous. A T&P valve releasing frequently could indicate excessive pressure or temperature — worth professional evaluation.
What to do: Check that inlet valves are fully open. If the T&P valve is releasing regularly, call a plumber — don't cap or disable the valve.
| Noise | Likely Cause | Urgency |
|---|---|---|
| Popping / Crackling | Sediment buildup | Address soon |
| Rumbling | Heavy sediment | Address soon |
| Knocking / Banging | Water hammer, expansion | Monitor / diagnose |
| Hissing / Sizzling | Sediment on element, leak | Check for leaks |
| Whistling | Restricted flow, T&P valve | Investigate |
Most water heater noises aren't emergencies. But they are signals. Here's when to move from "monitor" to "act":
If sediment is the cause (and it usually is), you have two paths:
Either way, addressing the noise now prevents bigger problems later.