An Engineer’s Guide to Boiler Noises: What Banging, Gurgling, and Whistling Really Mean

Is your boiler making strange noises? Banging, gurgling, whistling, humming, or clicking sounds can be alarming, but most have straightforward explanations. Some are harmless. Others need attention before they become expensive problems. This guide explains what each noise means and what to do about it.

What’s Normal?

All boilers make some noise. Here’s what you should expect from a healthy boiler:

• Soft whooshing when the burner ignites
• Gentle hum from the pump and fan while running
• Quiet clicking as the ignition sequence starts
• Occasional ticking from pipes expanding and contracting
• Water flow sounds when hot water taps are opened

These sounds should be subtle. You might hear them if you’re standing next to the boiler, but they shouldn’t be audible from other rooms or disturb your sleep.

When to pay attention: New noises, louder noises, or noises that have changed are worth investigating. Your boiler knows its normal, and changes often indicate something has shifted.

Banging and Knocking

Banging is one of the most alarming boiler noises, but it’s rarely dangerous. Here’s what causes it:

Kettling

The most common cause of banging. Limescale or sludge builds up inside the heat exchanger, restricting water flow. Water gets trapped, overheats, and turns to steam. The steam bubbles collapse violently, creating a banging or rumbling sound (like a kettle boiling).

Solution: A power flush can clear sludge and restore flow. In hard water areas like London, descaling the heat exchanger may also be needed. Prevention: annual servicing and a system filter.

Trapped air

Air pockets in the system can cause banging as water pushes past them. This is common after the system has been drained (for repairs) or if there’s a slow leak allowing air in.

Solution: Bleed your radiators to release trapped air. Start with radiators furthest from the boiler. Check system pressure afterwards and top up if needed.

Delayed ignition

If gas builds up before igniting, the eventual ignition can cause a bang or small explosion sound. This is more serious and usually indicates dirty or faulty burners, a blocked jet, or ignition timing issues.

Solution: Call an engineer. Delayed ignition needs professional diagnosis and can become a safety issue if ignored.

Water hammer

A sharp bang when taps or valves close suddenly. The momentum of flowing water creates a pressure wave that “hammers” the pipes. More common in older systems with high mains pressure.

Solution: Water hammer arrestors can be fitted. Adjusting mains pressure or valve closing speed may help. Often a plumber issue rather than boiler issue.

Thermal expansion

Pipes expanding and contracting as they heat and cool can knock against joists, floorboards, or clips. Usually a rhythmic ticking or knocking that follows heating cycles.

Solution: Often harmless. Pipes may need re-clipping with space to expand, or lagging to reduce noise transmission.

Gurgling and Bubbling

Gurgling sounds usually indicate air or gas in places it shouldn’t be.

Air in the system

The most common cause. Air enters through leaks, during maintenance, or through faulty components. You’ll often hear gurgling from radiators as well as the boiler.

Solution: Bleed radiators. If air keeps returning, there’s either a leak or a faulty component (like an automatic air vent) that needs attention.

Frozen condensate pipe

In cold weather, a frozen condensate pipe can cause gurgling as the boiler tries to push condensate through a blocked route. Often accompanied by a lockout and error code.

Solution: Thaw the pipe with warm water. See our frozen condensate guide for step-by-step instructions.

Low water pressure

If system pressure drops too low, air can enter the circuit. Gurgling may accompany other low-pressure symptoms like radiators not heating fully.

Solution: Check pressure gauge and repressurise if needed. If pressure keeps dropping, find the leak.

Pump positioned incorrectly

If the pump is mounted in a position where air can collect, you may hear gurgling from the pump itself. Some pumps have bleed screws to release trapped air.

Solution: Bleed the pump if accessible. If the problem persists, the pump position may need reviewing.

Whistling and Whining

High-pitched whistling often indicates restricted flow or component wear.

Kettling (again)

The same limescale/sludge buildup that causes banging can also produce a whistling or whining sound, especially in its early stages. It’s essentially water boiling in a restricted space.

Solution: Power flush and/or heat exchanger descale. See kettling section below.

Air in the system

Air passing through valves, pipes, or the pump can whistle. Often a high-pitched, intermittent sound.

Solution: Bleed radiators and check for sources of air ingress.

Pump speed too high

If the pump is set to a higher speed than necessary, water flowing too fast through pipes and valves can cause whistling.

Solution: Reduce pump speed. Most pumps have adjustable settings (1, 2, 3 or variable). An engineer can optimise this.

Partially closed valve

A valve that’s not fully open (thermostatic radiator valve, lockshield, or isolation valve) can cause water to whistle as it forces through the restriction.

Solution: Check all valves are fully open (or set correctly for TRVs). Listen to identify which valve is the source.

Faulty pump bearings

A worn pump can whine or screech, especially on startup. The bearings may be failing.

Solution: Pump likely needs replacing. A failing pump will eventually seize, so address it before you’re left without heating.

Humming and Vibrating

A low hum is often normal, but excessive humming or vibration needs attention.

Normal operation

The pump and fan both produce a gentle hum during operation. This is normal and shouldn’t be loud enough to notice from another room.

Pump vibration

If the pump isn’t mounted securely, or if its internal parts are wearing, it can vibrate against the casing or pipework. This creates a resonant humming that can travel through the house.

Solution: Check pump mounting. Anti-vibration brackets can help. If the pump itself is faulty, it may need replacing.

Fan vibration

The fan (which provides combustion air) can hum if bearings are wearing or if there’s debris on the blades.

Solution: Fan may need cleaning or replacing. An engineer can diagnose.

Electrical hum

Transformers, PCBs, and other electrical components can hum. A slight hum is normal; a loud or buzzing hum may indicate an issue.

Solution: If accompanied by other symptoms (error codes, erratic behaviour), call an engineer.

Loose casing or panels

Vibration from internal components can resonate through loose casing panels, amplifying the sound.

Solution: Check that all boiler casing panels are securely fitted.

Clicking and Tapping

Clicking is often completely normal.

Ignition sequence

A click-click-click followed by the burner firing is the normal ignition sequence. The spark electrode is creating sparks to ignite the gas.

This is normal. No action needed unless the clicking continues without the burner firing (ignition failure).

Expansion and contraction

As pipes heat up and cool down, they expand and contract. This can create ticking or clicking sounds, especially where pipes pass through walls, floors, or tight spaces.

This is normal. Can be reduced by ensuring pipes have space to move and using plastic clips rather than metal.

Motorised valves

Zone valves and diverter valves click when they open and close. A single click when heating starts or stops is normal.

Normal unless the clicking is continuous or accompanied by heating issues (one zone not working, no hot water, etc.).

Continuous clicking without ignition

If you hear repeated clicking but the boiler doesn’t fire, the ignition system is trying but failing. This usually triggers an error code.

Solution: Could be gas supply issue, faulty electrode, or other ignition problem. Check gas (do other appliances work?) and call an engineer if needed.

Dripping Sounds

Dripping inside or around a boiler always warrants investigation.

Internal leak

Water dripping from the heat exchanger, pump seals, or internal pipework will collect in the bottom of the boiler. You may hear dripping or see water underneath.

Solution: Call an engineer. Internal leaks need professional diagnosis and repair.

Pressure relief valve (PRV) discharge

If pressure gets too high, the PRV releases water through a discharge pipe (usually exits outside near the flue). You may hear dripping or see the external pipe wet.

Solution: Occasional discharge after overfilling is normal. Regular discharge indicates a fault. See pressure guide.

Condensate

Modern condensing boilers produce condensate (acidic water) as a byproduct. This drains through a pipe to a drain. Dripping sounds from condensate are normal.

Normal unless the condensate pipe is blocked or frozen (see frozen condensate guide).

External leak

Leaks from pipework, valves, or connections near the boiler can sound like they’re coming from the boiler itself.

Solution: Trace the source and repair. Don’t ignore any visible leaks.

What Is Kettling?

Kettling deserves its own section because it’s one of the most common and misunderstood boiler problems.

The cause

Over time, limescale (in hard water areas) and magnetite sludge (black iron oxide from corroding radiators) accumulate inside your boiler’s heat exchanger. This restricts water flow through the narrow passages.

When water can’t flow freely, it overheats in localised spots. The water boils, creating steam bubbles. When these bubbles hit cooler water, they collapse violently. This produces the rumbling, banging, or whistling sound that resembles a kettle boiling.

The symptoms

• Banging, rumbling, or whistling from the boiler
• Boiler cutting out on overheat
• Radiators slow to heat or not as hot as they used to be
• Hot water temperature fluctuating
• Higher gas bills (boiler working harder)

Why it matters

Kettling isn’t just annoying. It:

• Stresses the heat exchanger, potentially shortening boiler life
• Reduces efficiency (costs you money)
• Can trigger safety lockouts
• Gets worse over time if ignored

The solution

Power flush: A power flush clears sludge from the entire system, including the heat exchanger. This is the most effective treatment for kettling caused by magnetite buildup.

Descale: In hard water areas (including most of London), limescale may also need chemical descaling, specifically targeting the heat exchanger.

Prevention: Annual servicing, a magnetic system filter (like MagnaClean), and chemical inhibitor will prevent future buildup.

London note: London has hard water. If you’ve lived in your property for years without a system filter or regular flushing, there’s a good chance you have limescale buildup. Kettling is extremely common in London properties.

Noises from Radiators

Sometimes the noise seems to come from radiators rather than the boiler.

Gurgling from radiators

Air trapped in radiators. Bleed the affected radiator using a radiator key.

Clicking or ticking

Thermal expansion. Radiators and pipes clicking as they heat up and cool down. Normal.

Banging from radiators

Could be water hammer, or pipes knocking against floors/walls as they expand. Check pipe runs for adequate clearance.

Hissing

If a TRV (thermostatic radiator valve) is partially open, water forcing through can hiss. Try opening the valve fully or turning it down. If persistent, the valve may need replacing.

Cold radiator with noisy boiler

If a radiator isn’t heating and the boiler is making noise, there may be a circulation problem, a stuck valve, or an airlock in that specific radiator circuit.

When to Call an Engineer

Some noises you can address yourself. Others need professional attention.

DIY fixes

• Bleeding radiators to release trapped air
• Repressurising if pressure is low
• Thawing a frozen condensate pipe
• Checking valves are fully open
• Tightening loose casing panels

Call an engineer for

• Kettling sounds (banging/whistling from the boiler itself). Needs diagnosis and likely a power flush.
• Delayed ignition (bang when boiler fires). Safety issue.
• Continuous clicking without ignition. Ignition system fault.
• Internal dripping or visible leaks. Needs repair.
• PRV discharging repeatedly. Underlying pressure issue.
• Pump whining or screeching. Likely failing.
• Any noise accompanied by error codes. Let the code guide diagnosis.
• Noises that have suddenly changed or worsened. Something has shifted.

Frequently Asked Questions