I know this is a common question here, but I have looked and failed miserably to find a fix for my problem. I have a 20+ year old Potterton Netaheat, gravity fed, open vent CH & HW system in a hard water area. I moved in 3 years ago and found my boiler was noisey - sounds like knocking on drums and bubbling within a minute or two of the boiler coming on, worse when just the HW is on. I am layman and asked several local plumbers what could it be. The general view was limescale and deposit build up causing kettling. Various advice ranged from leave it alone to change the boiler. I recently opted for a powerflush. All seemed to go well for about a day - the banging ceased and we overslept it was so quiet. Then it became obvious the noise was returning - worse than ever now - and there is a huge amount of air collecting in the bathroom towel rail [that runs off the HW system] as you can hear bubbles in it like a scuba aqua lung and I have to vent out about a litre twice a night - there's hardly any build up in the radiators which are all nice and hot. It seems as if air is getting in somewhere and not escaping from the boiler. I've tried suspending a glass of water on the end of the vent, as your pages suggest, but no water goes up, or air or water comes out. I can't identify anywhere to vent the boiler. It never used to be like this! Clearly I need to get an expert in, but what could be going wrong?