Why does a radiator make a knocking or banging noise?

Clicking and cracking — thermal expansion

The most common sound. When the heating water heats up, the metal expands and moves against a bracket or a floor duct. The sound is cyclical, follows the heating turning on and off, and disappears once the temperature is steady.

This isn't a fault, but it can be reduced if there's expansion room in the pipe ducts and the brackets aren't clamped too tightly.

Gurgling and bubbling — air in the system

A gurgling, flowing-stream-like sound almost always means air. It's usually accompanied by the top of the radiator staying cold.

This is normal for a short while after the system has been refilled, but if air keeps coming back, there's a leak somewhere or the automatic air vent isn't working properly.

A sharp knock or bang — water hammer

A single loud metallic bang in the pipework occurs when a moving column of water is stopped abruptly. In apartment blocks this often happens with valves closing sharply or when a pump is running.

Water hammer isn't just cosmetic — the impacts strain the connections and are one reason an old threaded joint eventually starts to leak.

• The bang always happens during the same action — check the valves and pump settings
• The bang is random — more often related to system pressure or air
• The bang has grown stronger over time — needs a technician's assessment

Rattling and scraping — debris or scale

If the sound is more like sand or gravel moving, there's debris circulating in the system: magnetite, rust and sediment from old pipes.

In this case uneven heating usually accompanies the noise — some radiators are hot, others lukewarm. This is a typical sign the system needs flushing, which our team can arrange as part of general plumbing work.

When to call a technician

Call a technician if the noise is accompanied by a drop in pressure, visible moisture, uneven heating, or if the bangs are strong enough to make the pipework visibly move.

If the noise is just clicking and all the radiators heat evenly, there's no need for urgent action — a pre-heating-season check is enough.

Frequently asked questions