What frequency? Hourly, Daily, Weekly?
is it a single Bang, or is it repeated if so Rapid? Slow?
Could be one of several causes, from Kettling in the heat exchanger, boiler thermostat causing overheat Baffles tight and banging when they get hot and release, or possible a Combustion problem such as Delayed Ignition, too much air, too little air etc etc
More info would be useful!