Selgas
I am not too sure about the model you have there but from your very clear description of the problem it seems that the air gas mixture is incorrect at the main burners and resulting in what we call “flash Back” that is the gas from the particular burner is lighting at the injector rather than the burner outlet – this makes a loud roaring noise when it happens. Not sure why it would suddenly happen after you have just changed a thermocouple but if you have altered the pilot and relocated it incorrectly at the wrong part of the main burners you could cause the same result to happen. Hope this helps.
