Sounds like explosion ignition if the front of the boiler rattles also, might be a problem with spark electrode whereby the gas builds up in the combustion chamber then explodes into ignition or could be partially blocked burner, needs looking at.
make sure the black condensate trap is clear, underneath the boiler bit of a pain to take out though, but can be done, also check the bottom of the combustion chamber is clear by removing the front metal cover strip, 3 screws, be carefull if there is any water build up it can swamp the electrics box fitted below (good design this one) after that Ign Electrode to check out RGI JOB THESE
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