Kettling is one common cause. This happens when heat builds faster than water can move away from the heat exchanger. The sound can resemble banging, rumbling, or a low vibrating noise. Restricted flow is often the underlying reason.
Low system pressure can make this worse. When pressure drops, water doesn’t circulate smoothly, allowing heat to build unevenly. This is particularly common in older systems or where pressure loss has been ongoing.
Another contributor is trapped air or sludge. Both reduce internal flow and cause turbulence as water is forced through narrower pathways. If radiators also heat unevenly, that link is explained here: Radiators Heat Unevenly.
If the boiler noise coincides with short heating cycles, this pattern is covered in detail here: Why Your Boiler Keeps Turning On and Off.
Understanding when boiler noises matter and when they don’t is part of keeping a system running efficiently, which is explained fully here: How to Keep a UK Home Warm for Cheap.
If the noise is new or worsening and you’re unsure where to start, this diagnostic page helps narrow it down logically: House Cold Diagnostic Tool.