broag noisey start up

Noisey start up on ignition ( trumpet like sound) i attended a call 'noisey boiler' customer demostrated the problem which was occasionaly severe. After chaning door seal, burner seal, testing Co2, tring spare gas valve and fan checking flue seals, and flue, I noticed a severe disturbance in the Co2 trap. I disconected the top trap connection and noise stopped . I suspect the condensate was partialy blocked causing a back pressure to force up back through the burner. look out for it guys................