Potterton Prima C

[cowdrayplumber]

Hi there, I'm fairly new to the gas world and have come across a problem on a potterton Prima c that I have not experienced before. When the boiler is running the flame makes a 'popping' noise. The flame picture looks ok. I have gas rated the appliance and it as it should be as is the gas pressure. If you lift the casing whilst lit the popping stops but then starts again as soon as the casing is lowered again. The room has adequate ventilation. If anyone could offer some advice it would be much appreciated.