I have a vented heating and hot water system with a hot water cylinder. The system was drained down so the heat exchanger could be cleaned, but since re-filling there seems to be air getting into the system causing the boiler to make a whining sound and occasionally banging noises. The water heats ok and the heating gets hot. I have tried to bleed the air out from the 2 drain valves near the boiler, and flushing the heating system by opening the drain valve at the lowest point but there seems to be an endless amount of air coming through. I've added noise reducer and leak sealer but these don't seem to have had any effect. I have tried putting a cup of water over (or under) the vent that goes into the header tank and it's not pulling in any air from there. I've bled the pump. Can anyone give me any ideas where the air could be getting in?