Viessmann 100-w 35kw combi loud bang

[Plumber69]

Getting quite a loud noise, delayed ignition. I cleaned the ignitor and screens. This seemed to help as the bang is less frequent and quieter. Any Idea's?

 

[Fitter30]

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White substance is it kind of fluffy or baking soda? Can be caused by a chemical in the air like in a beauty salon being burned.