Popping, rumbling, or knocking from a water heater almost always means sediment buildup—minerals that settle on the tank bottom, trap water underneath, and create noise when that water turns to steam. Flushing the tank usually solves the problem. Other noises have different causes and solutions.
Water heaters are usually silent workhorses. When they start making noise, they’re trying to tell you something. Here’s how to decode the sounds and take appropriate action.
The Most Common Noise: Popping/Rumbling
This is what most people describe, and it has one main cause.
What’s Happening
Minerals (calcium, magnesium, lime) in your water settle at the bottom of the tank over time, forming a layer of sediment. When the burner heats the tank:
- Water trapped under the sediment gets superheated
- It turns to steam
- Steam bubbles force their way up through the sediment
- This creates popping, cracking, or rumbling sounds
The sound might be louder when the heater first fires up, then quiet down, or it might be continuous during heating cycles.
How Serious Is It?
Sediment buildup is:
- Not immediately dangerous: Your water heater won’t explode
- Bad for efficiency: The burner works harder to heat through sediment
- Damaging over time: Hot spots can weaken the tank bottom
- Reducing capacity: Sediment takes up space meant for water
The Fix: Flush the Tank
Flushing removes loose sediment. Here’s how:
For gas water heaters:
- Turn the gas control to “Pilot”
- Connect a garden hose to the drain valve at the bottom
- Run the hose to a floor drain, outside, or into buckets
- Open a hot water faucet in the house (allows air in)
- Open the drain valve
- Let water flow until it runs clear
- Close everything, let tank refill, restore gas to normal
For electric water heaters:
- Turn off power at the circuit breaker
- Follow the same drain procedure
- Restore power only after the tank is completely full
If little or no water comes out when you open the drain valve, the valve itself may be clogged with sediment. Try opening it all the way and letting it run, or carefully use a screwdriver in the opening to break up debris. If the valve is completely blocked, you may need professional help.
If Flushing Doesn’t Help
Heavily calcified sediment won’t flush out easily. Options:
- Professional descaling with acid solution
- Multiple flush cycles with full tank drains
- Tank replacement if damage has occurred
Knocking or Hammering
Different from the popping of sediment, this is often “water hammer.”
What’s Water Hammer?
When water flow is suddenly stopped (like a quick-closing valve), the momentum creates a pressure wave that bangs through pipes. It sounds like someone hitting the pipes with a hammer.
Causes Near Water Heaters
- Washing machine or dishwasher valves closing quickly
- Single-lever faucets shutting off
- Toilet fill valves stopping suddenly
- Sprinkler systems activating
Fixes for Water Hammer
Secure loose pipes: Vibrating pipes amplify the sound. Add pipe straps where pipes pass through floor joists or walls.
Install water hammer arrestors: These devices absorb the pressure spike. Install near the source (washing machine, etc.).
Check expansion tank: Homes with closed-loop plumbing systems need an expansion tank on the water heater. If it’s waterlogged, water hammer worsens.
Lower water pressure: High pressure (above 80 PSI) makes hammer worse. A pressure reducing valve helps.
High-Pitched Whining or Screeching
This noise usually indicates a flow restriction somewhere.
Possible Causes
Partially closed valve: Check that inlet and outlet valves are fully open
Kinked or restricted supply line: Look for flexible connectors that are bent
Failing inlet valve: The valve may be partially seizing
Electric element scale buildup: Scaling on elements can cause whining as water forces past
Fixes
- Verify all valves are fully open
- Check supply lines for kinks
- For recurring whining, the inlet valve may need replacement
- Descaling electric heater elements can help
Hissing or Sizzling
These sounds have different meanings for gas vs. electric heaters.
Gas Water Heaters
Normal hissing: You may hear the gas burner, especially with newer high-efficiency models. This is normal.
Sizzling: If water is dripping onto hot burner components from condensation or a leak. Check for:
- Condensation (common during startup)
- Tank leak (serious—check for water pooling)
- T&P valve discharge (check if the valve is dripping)
Electric Water Heaters
Sizzling: Usually a lower heating element with sediment buildup. The element gets buried in sediment and superheats the surrounding water.
Crackling: Similar cause—element cycling through heavy sediment.
Fix: Flush the tank or replace the lower element if damaged.
If your gas water heater sizzles and you see water under the tank, this could indicate a tank leak. Check the drain valve (may be dripping), T&P valve (may be discharging), or tank seams. Tank body leaks usually mean replacement is needed.
Ticking or Tapping
Often related to thermal expansion rather than a problem.
Heat Trap Nipples
Many modern water heaters have heat trap nipples on the inlet and outlet that prevent heat loss. These contain small balls or flappers that can tick during temperature changes.
Fix: If annoying, heat traps can be replaced with standard nipples, though you’ll lose some efficiency.
Pipe Expansion
Copper and other metal pipes expand when hot water flows through them. If pipes are tightly secured, they may tick against mounting straps or wood.
Fix: Loosen pipe straps slightly or add felt padding where pipes contact other surfaces.
Vibrating or Humming
Electric Water Heater Humming
The heating elements may vibrate at electrical frequency, causing a hum. This is usually normal and not harmful.
If it’s new or getting louder:
- Elements may be loose—can be tightened
- Sediment around elements—flush the tank
- Failing element—test and replace if needed
Gas Water Heater Vibration
Less common, but possible causes:
- Draft issues affecting the burner
- Exhaust vent vibration
- Improper combustion
Have a professional check gas heaters with unusual vibration or humming.
When Noise Indicates Real Problems
Some noises warrant immediate attention:
Continuous boiling or rumbling that’s getting louder over time suggests significant sediment and potential tank stress.
Noise accompanied by water leaks may indicate tank failure.
Noise after T&P valve discharge suggests dangerous pressure or temperature buildup—call a professional immediately.
New sounds from a recently installed heater may indicate installation issues.
Preventive Maintenance
Stop noises before they start:
Annual Flushing Schedule
- Put it on your calendar
- Take 30 minutes to flush sediment
- Check the anode rod while you’re at it (sacrificial rod that protects tank)
Water Softener
If you have hard water:
- A water softener dramatically reduces mineral buildup
- Less sediment = quieter operation = longer tank life
- May not be practical for everyone but very effective
Lower Temperature
Higher temperatures accelerate mineral precipitation:
- 120°F is usually adequate
- Lower temps mean less sediment formation
- Saves energy too
Expansion Tank
If your home has a closed plumbing system:
- An expansion tank absorbs thermal expansion
- Reduces stress on the water heater
- Prevents water hammer
Sound Summary Table
| Sound | Likely Cause | Action |
|---|---|---|
| Popping/rumbling | Sediment buildup | Flush tank |
| Knocking/hammering | Water hammer | Install arrestors |
| Whining/screeching | Flow restriction | Check valves, lines |
| Hissing (gas) | Normal burner sound | None needed |
| Sizzling with water | Leak onto hot surfaces | Investigate source |
| Ticking | Thermal expansion | Usually normal |
| Humming (electric) | Element vibration | Usually normal |
When to Call a Professional
- Noises accompanied by leaks
- Gas smell with any noise
- T&P valve repeatedly discharging
- Unable to flush due to clogged drain valve
- Noise persists after flushing
- Any situation you’re uncomfortable handling
Next Steps
If your water heater is noisy and also not heating well, see Water Heater Not Hot Enough. For general plumbing maintenance including water heater care, check our Plumbing Fixes Guide.