Why Does My Faucet Make A Knocking Sound And How To Fix Water Hammer

If you've ever turned off a faucet or washing machine and heard a loud bang or series of knocks in your pipes, you're experiencing what's known as water hammer. This phenomenon isn't just annoying—it can damage your plumbing system over time if left unaddressed. The good news is that water hammer is both diagnosable and fixable, often without requiring expensive professional help. Understanding the mechanics behind it and knowing how to respond can save your pipes, appliances, and peace of mind.

What Is Water Hammer?

Water hammer occurs when flowing water is suddenly stopped—typically by a fast-closing valve such as those in dishwashers, washing machines, or even standard faucets. When this happens, the momentum of the moving water creates a pressure surge that travels through the pipes, causing them to vibrate, shake, or knock against surrounding structures like wood framing or walls.

This shockwave can generate pressures significantly higher than normal operating levels—sometimes exceeding 1000 psi in extreme cases—putting stress on joints, fittings, and valves. Over time, repeated occurrences can lead to leaks, pipe joint failure, or premature wear on appliances.

“Water hammer is one of the most common yet overlooked causes of long-term plumbing damage in residential homes.” — Robert Langford, Master Plumber and Plumbing Systems Instructor

Common Causes of Knocking Sounds in Faucets

The knocking noise itself may vary—some describe it as a thud, others as a rapid banging—but the underlying cause usually traces back to one or more of these factors:

• Rapid-closing valves: Modern appliances use solenoid valves that shut off water almost instantly, increasing the risk of shockwaves.
• Air chambers that have waterlogged: Vertical pipes near fixtures act as shock absorbers, but they can fill with water over time, losing their cushioning effect.
• Loose pipe straps or hangers: If pipes aren’t securely fastened, they can move and strike framing when pressure surges occur.
• High water pressure: Pressure above 80 psi increases the severity of water hammer effects.
• Lack of water hammer arrestors: Older homes or DIY installations may not include these essential dampening devices.

Tip: If the knocking only happens with specific appliances (like your washing machine), the issue likely stems from that fixture’s valve or dedicated supply line.

How to Diagnose Water Hammer at Home

Before attempting any repairs, confirm that what you’re hearing is indeed water hammer and not another plumbing issue like loose fittings or sediment buildup. Follow this diagnostic process:

1. Identify the trigger: Does the noise happen when you turn off a faucet, flush a toilet, or stop a washing machine? Water hammer typically follows quick shutoffs.
2. Note the location: Is the sound coming from behind a wall, under a sink, or near an appliance? Localized noise helps pinpoint the source.
3. Check for patterns: Does it occur more frequently during peak water usage times? High demand can amplify pressure fluctuations.
4. Test water pressure: Use a pressure gauge attached to an outdoor spigot. Normal pressure ranges between 40–80 psi. Anything above 80 psi should be addressed.
5. Drain the system: Temporarily relieve air chamber saturation by draining all water from the plumbing (more on this later).

Mini Case Study: The Noisy Washing Machine

Homeowner Lisa M. noticed a loud banging every time her front-loading washing machine finished its cycle. Initially dismissing it as normal operation, she began to worry when the noise grew louder and started vibrating nearby cabinets. A plumber confirmed water hammer caused by a combination of high household pressure (92 psi) and missing water hammer arrestors on the laundry lines. After installing arrestors and adjusting the pressure regulator, the knocking ceased entirely—saving her from potential pipe damage down the line.

Step-by-Step Guide to Fix Water Hammer

Fixing water hammer involves restoring air cushions, securing pipes, or installing mechanical dampeners. Here’s how to do it yourself:

Step 1: Shut Off Main Water Supply

Locate your home’s main water shut-off valve—usually near the water meter or where the line enters the house. Turn it off completely. Open the highest faucet in your home (like a second-floor bathroom) to release system pressure.

Step 2: Drain the Plumbing System

Open all faucets, starting from the highest point and working downward. Flush toilets and drain appliances like water heaters if necessary. This removes water from vertical air chambers, allowing them to refill with air when water returns.

Step 3: Restore Air Chambers

Once drained, close all faucets and slowly turn the main water supply back on. As water re-enters the system, air will naturally be trapped in the top of vertical pipes, recreating the cushion needed to absorb shock. This simple reset fixes many cases of mild water hammer.

Step 4: Secure Loose Pipes

If knocking persists, inspect accessible pipes in basements, crawl spaces, or under sinks. Look for areas where pipes rest directly against wood or metal. Use foam padding and secure them with proper pipe straps spaced every 4–6 feet. Never allow copper or PEX lines to touch structural elements directly.

Step 5: Install Water Hammer Arrestors

For persistent issues—especially near washing machines, dishwashers, or ice makers—install mechanical water hammer arrestors. These devices contain a sealed air bladder that compresses upon pressure spikes, absorbing the shock.

1. Turn off water and power to the appliance.
2. Cut into the supply line using appropriate tools (copper requires soldering; PEX uses crimp rings).
3. Solder or connect the arrestor in-line before the appliance valve.
4. Test the system after reassembly.

Pre-assembled arrestor kits are available for $20–$40 and fit standard 1/2-inch connections. They require no maintenance and last 10+ years.

Tip: Install arrestors as close as possible to the source of the hammer—usually within 12 inches of the appliance valve—for maximum effectiveness.

When to Call a Professional

While many water hammer issues can be resolved DIY-style, some situations warrant expert intervention:

• Visible leaks or bulging pipes after hammer events
• Chronic knocking despite multiple fixes
• Water pressure consistently above 80 psi
• No accessible shut-off valves or complex piping layouts

A licensed plumber can install a whole-house pressure-reducing valve (PRV), add permanent arrestors, or redesign problematic sections of your plumbing. The average cost for professional diagnosis and repair ranges from $150 to $400, depending on complexity.

Do’s and Don’ts of Water Hammer Prevention

Do	Don’t
Regularly check household water pressure	Ignore persistent knocking sounds
Drain the system annually to refresh air chambers	Use duct tape or zip ties to secure pipes
Install arrestors on automatic appliances	Assume new plumbing is immune to water hammer
Secure all exposed pipes properly	Over-tighten fittings, which can crack pipes
Monitor changes after renovations or new appliance installs	Delay repairs—early action prevents costly damage

Frequently Asked Questions

Can water hammer damage my plumbing over time?

Yes. Repeated pressure surges weaken joints, erode seals, and can eventually cause pinhole leaks or burst pipes. Chronic water hammer shortens the lifespan of water heaters, washing machines, and other pressurized appliances.

Is water hammer dangerous?

While not immediately hazardous like a gas leak, untreated water hammer poses long-term risks. In rare cases, extremely high pressure spikes can rupture pipes, leading to water damage or mold growth inside walls.

Why did my faucet start knocking suddenly?

Sudden onset often indicates a change: a new appliance installation, recent plumbing work, or the saturation of air chambers. It can also follow municipal water system adjustments that affect incoming pressure.

Conclusion: Take Action Before Damage Sets In

The knocking sound from your faucet might seem minor, but it’s a warning sign your plumbing system can’t afford to ignore. Water hammer isn’t just noise—it’s physics in action, and each bang chips away at your pipes’ integrity. By understanding the root causes and taking proactive steps, you can eliminate the disturbance and extend the life of your entire plumbing network.

Start with simple fixes like draining the system and securing loose pipes. For recurring issues, invest in water hammer arrestors—they’re inexpensive insurance against future headaches. Whether you handle it yourself or call in a pro, addressing water hammer now prevents costly repairs down the road.

💬 Have you fixed water hammer in your home? Share your experience or ask questions in the comments—your insight could help someone avoid a plumbing disaster!