There’s something unnerving about stepping into your home and hearing a banging noise coming from your water heater. Water heater making knocking noise? Read on…

What started as an occasional tap may grow into a persistent thud echoing through the walls. 

In this article, we will help you to understand; what a knocking water heater means, when it’s harmless, when it’s serious, and how to fix it (or call in a pro).

Skip to:

• Water heater noises explained
• Reasons for water heater making knocking noise
• How to diagnose
• How to fix
• FAQs
  

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Water heater noises explained

Your water heater (tank-type) can produce all sorts of noises. Some are normal; others signal a problem. According to plumbing experts:

🔊 Popping or rumbling is often caused by sediment buildup at the bottom of the tank.

🔊 Crackling or hissing is common in electric heaters when the heating element is covered by sediment or blocked

🔊 Ticking or clicking could be simply metal expanding / contracting, or due to thermostat / heating element cycling.

🔊 Banging or hammering that is more severe-sounding could be sediment, but also pressure issues, piping issues.

When your water heater is making a knocking noise, you’re likely hearing something in the rumbling or banging category. Let’s dig a little deeper.

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Reasons for water heater making knocking noise

If your heater is making a knocking or rumbling sound (sometimes described like a pot of boiling water, or as if some internal piece is striking the tank), there are several possible causes. 

The most common ones are:

👉 Sediment buildup at the bottom of the tank

Probably the single most frequent culprit. Over time, minerals (especially in hard-water areas) drop out of suspension and collect at the bottom of the heater tank, forming a layer of sediment or scale. 

Here’s what happens:

• The heating element or burner is typically at the bottom of the tank, underneath or near the sediment
  

• Water gets trapped beneath the sediment layer. When that water heats up, it forms steam bubbles or boils locally. Those bubbles fight their way through the sediment layer and ‘pop’ or ‘knock’ as they escape.
  

• The films call it ‘mini-explosions’ or bubbling under the mineral layer, and the tank then transmits the sound of those bubbles bursting or pushing through.The result? A knocking or rumbling noise. The knocking noise may be due to your tank having less volume. Steam bubbles get trapped instead of escaping as they are meant to. When they rattle around, they create this noise.

• Beyond the noise, this layering reduces the effective heating capacity (the heater works harder) and can lead to overheating of the tank or the heating element.
  

👉 Water hammer or pressure surges

Another possible cause, especially if the knocking occurs in the piping rather than ‘inside’ the tank sound. 

When water is flowing and suddenly stops or changes direction (for example, solenoid valves, washing machine shut-off, or high pressure), it creates a shock wave in the plumbing system – commonly called ‘water hammer’. 

In a water heater context:

• If water is rapidly entering the tank or being shut off, pressure surges can make the tank or pipes vibrate or knock.
  

• Loose piping supports or mounts can worsen the effect (pipes moving, hitting framing).
  

• Note: This is more about piping than the tank’s internal workings, but from the homeowner’s ear, it may sound as though the heater is knocking.
  

👉 Thermal expansion / contraction & loose parts

Metal expands when heated and contracts when cooled. The tank shell, connections, piping, insulation – all may shift. Some knocking could simply be the tank or its components flexing. 

Also:

• Loose valves, dip tubes, mounts, or heating elements may vibrate or hit other parts when the heater turns on or off. 

• Corrosion or worn parts may add to the movement, creating noise.
  

👉 High internal pressure or malfunctioning relief valve

Less common for knocking but still possible: If the internal pressure is too high (thermal expansion of water, trapped air, stuck relief valve) then the heater might knock as pressure bursts through sediment or escapes abruptly.

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How to diagnose 

Here’s a step-by-step guide you (or a plumber) can follow to identify what’s going on.

Step 1: Note when the noise happens

Ask yourself:

• Does the knocking happen whenever the heater cycles on?
  

• Does it occur only when hot water is running (or after a long idle)?
  

• Does it happen at specific times (e.g., first use in the morning)?
  

• Is it constant, intermittent, loud, subtle?
  

• Any other symptoms: water appearing rusty, less hot water, pressure release valve dripping, tank sweating, external piping vibrating?
  

Step 2: Visual inspection

• Check the area around the tank for visible leaks, corrosion, loose piping or clamps, flexing pipes.
  

• Listen whether the noise appears to come from inside the tank (deep rumble) or from the pipes/valves outside.
  

• Check the temperature setting → very high temperature may create more intense cycles and expansion.
  

• Check water pressure: unusually high pressure may cause more noise.
  

• If you live in a hard-water area, consider the likelihood of sediment buildup.
  

Step 3: Flush the tank (if sediment suspected)

Since sediment is the most common cause, flushing the tank is a key diagnostic and remedial step. Generic steps:

1. Turn off the power/gas supply to the heater.
   
2. Turn off the cold water supply.
   
3. Connect a garden hose to the drain valve at the bottom of the tank, direct to a safe drain.
   
4. Open the drain valve and let the tank flush until clear water flows. You may need to partially open the cold supply to stir sediment up.
   
5. Close the valve, refill the tank, restore power/gas, wait for normal operation.
   
6. Observe whether the knocking noise disappears or significantly reduces.
   

If the noise goes away after flushing → sediment was likely the main cause. If not, continue investigating.

Step 4: Check piping supports, pressure, water hammer

• Look at the piping near the heater – are pipes tightly secured or rattling when the heater fires up?
  

• If pipes are knocking or vibrating, it might be water hammer or expansion of piping. Installing water hammer arrestors, securing pipes, or pressure-reducing valves may help.
  

• Check pressure relief valve: lift the test lever and see if water discharges. If it’s stuck or faulty it may cause pressure issues.
  

• Check temperature setting: excessive temperature increases stress. Keeping thermostat at around 120-125°F (49-52 °C) is typical.
  

Step 5: Inspect for loose components or ageing parts

• Inside the tank (or for electric models) check heating elements, anode rods, dip tubes for condition. A worn anode rod can lead to corrosion, which exacerbates noise issues.

• For gas units, check ignition, burner, venting assembly – sometimes a ‘pop’ when burner ignites is normal, but a consistent loud bang might mean delayed ignition or gas valve issues.
  

• For older tanks, check for rust, pitting, weak shell – a banging may signal structural stress.
  

Step 6: Decide DIY vs Professional

If flushing the tank removed the noise and all piping and pressure look fine – you may have resolved it yourself.

However, call a licensed plumber if:

• There’s visible corrosion/leakage
  

• Pressure relief valve is dripping or stuck
  

• The knocking is extremely loud or seems to be getting worse
  

• You’re unsure about gas valve or burner issues (safety hazard)
  

• The heater is near end-of-life and other signs of failure are present
  

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How to fix

Fixing (based on cause):

🛠️ Sediment buildup

Flush the tank annually or bi-annually (in hard-water areas). Vacuuming or more intensive descaling may be needed for heavy buildup. 

🛠️ Water hammer/pressure surges

Install or service a water hammer arrestor, check and possibly reduce water pressure with a pressure-reducing valve, secure piping.

🛠️ Loose piping/insufficient supports

Tighten or replace pipe straps, ensure pipes are clear of contact with framing/wood, apply insulation where needed. 

🛠️ Excessive temperature/pressure

Lower thermostat to around 120-125°F. Inspect or replace the temperature and pressure relief valve.

🛠️ Corroded or worn parts (anode rod, tank, burner)

Replace anode rod every 4-5 years (especially in hard water). If tank is rusted or venting is compromised, consider replacing heater.

Prevention & Maintenance:

✅ Annual flush

Even if no noise, flushing the tank yearly prevents sediment accumulation.

✅ Water quality matters

If you live in a hard-water region (high in calcium/magnesium), consider installing a water softener to reduce sediment load on the heater.

✅ Check pressure relief valve and temperature settings

Make sure the T&P relief valve is functioning and the temperature is not set too high.

✅ Secure piping and support

During annual check, make sure all the heater’s connected pipes and supports are in good condition and not banging against framing.

✅ Inspect the anode rod

It might not cause a knocking noise directly, but a failing anode rod can lead to corrosion that undermines the tank structure.

✅ Stay alert for other warning signs

Reduced hot water, rusty water, leaks, constant cycling – these might accompany or precede noises. Ignoring them shortens heater lifespan.

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Frequently asked questions (FAQs)