My combi boiler is making backfiring sounds. Help!?

When I use hot water at a certain pressure, the boiler kicks in as normal but a few seconds later I hear a loud bang like a car backfiring. Its coming from the boiler. There is no other problem as the water remains hot but I'm guessing the 'bang' is not a good thing. I had some parts fixed on the boiler about 3 weeks ago as I have a leaking overflow pipe! Any ideas please.