these air bubbles sound as though there is an army of aquatic dwarfs inside the pipes and knocking h... out of them. The air may only be drained off by opening the hot water tap in the upstairs bathroom & when this is done the physical impact of these fast-moving bubbles can be felt easily by gripping the barrel of the tap. After about twenty seconds the water flow is smooth and untroubled. I have noticed that when the downstairs boiler switches off, after a few seconds there is a clearly audible sound just like a long in-drawing of breath ..... could this be the boiler sucking in air as it cools down?
When it occurs, the pipe-banging is easily heard inside the house but it is outside, immediately under the balanced-flue boiler's grill, that the sound is loudest. Does it sound like a boiler problem to you?
Hwyl fawr! John Watkins