why is my boiler whistling

Limescale buildup on the heat exchanger causes water to overheat locally, producing steam bubbles that create whistling or kettling sounds.

Trapped air in the system can also cause whistling as water flows past air pockets. Bleed your radiators to release trapped air.

A faulty circulator pump with worn bearings can whistle or whine. If the pump is overheating or making noise, it may need replacement.