It might be boiler kettling.
Possibly related to limescale deposits.
AI search result on kettling (I knew about it before doing a search)
"Boiler kettling is a term used to describe the noise produced by a boiler that resembles the sound of a boiling kettle. This noise occurs when there is an obstruction or restriction in the flow of water through the boiler system, leading to the boiling and bubbling of water, which results in the kettling noise.
Common causes of boiler kettling include limescale build-up, which is more prevalent in hard water areas, and issues with the thermostat or boiler pump, which can lead to overheating.
Limescale can accumulate inside the pipes and other components of the boiler, slowing down the water flow over the heat exchanger and causing excessive heating."