alpha cb 24?

can any one help , heating works fine, call for hot water , fires up for about 10 seconds then shuts off , will then fire up again after about 15 seconds run for about 10 seconds and so on, before it shuts off it makes a sound like an aeroplane landing. i have been told it may be the hot water heat exchanger, any ideas anyone, i have replaced the overheat sensor tec.