How can I can fix my Glow worm 45 Space Saver 45-60B Boiler?

Everything fires up quite normally. As the system warms up, the boiler makes a chuffing noise rather like a steam engine. When the operating temperature is reached,the noise generally stops, but will often start again intermittently when the boiler calls for heat. It only occurs while the burner is lit. The higher the boiler stat setting, the greater the problem. Any ideas.