When the hot water or central heating system is switched on and the valve in the airing cupboard opens there is a whooshing noise . Eventually the noise goes away and it all seems to be working until I start to get some banging noises from the boiler. At this point I have to switch the boiler off and switch it on again which then results in a sudden rush of water into the boiler. It all then starts working properly. It cant be air in the system cos I have an automatic air vent in the airing cupboard. I also have an air separator. The boiler has been serviced and the system descaled Any ideas?? My boiler is an Ideal Classic