Baxi duotec combi 28 he trickling water from condensate overflow

Hi got the above boiler few years old. Occasionally I get a " banging" from the boiler when the hot water is running. This is cured by removing and cleaning out the secondary heat exchanger. Today same symptoms, but when I checked the boiler I could hear trickling water and the pressure had dropped. I closed the main house CH valves and tried to re-pressurise. Still heard trickling water which was coming from the condensate trap overflow pipe outside (white plastic) The boiler empties from 2.5 pressure to zero in about 90 seconds. Any ideas please guys ? Thanks in advance....