Could be airlock but sludge is also a possibility causing local overheating. Clanking is caused by steam bubbles forming and collapsing. (Boiler is still hot, water circulation has stopped) Causing the pump to run on after the boiler ceases firing is a "fix". (You can try this to prove the cause)
Boiler needs to be flushed out, ideally by removal and hose pipe applied outside in the garden.
Most likely in an old system with no water treatment.