Boiler creating air?

Need some advice concerning my central heating/water boiler.
The system is fired by a Glow-worm Fuelsaver 30-40 about 25 years old.
When switched to just water (no heating) periodic banging occurs caused by air moving through pipes. Switch to both water and heating the problem goes away (does this mean that the pump is OK?). It seems as though the boiler is creating the air. No work has been carried out any where in the system and what little air there has been has been bled several times through the radiators. Is my boiler on the way out?
Any help or advice would be gratefully received, Thank you.