Bosch Greentherm T990 SE 199 - Loud Click When Firing

[Rossroto]

Hi there - I'm trying to troubleshoot why this Bosch Greentherm T990 SE 199 is making a loud click/clunk noise when firing. Here's a video with the noise. Is this being caused by a defective/obstructed valve? Someone on reddit suggested it could be a water adjustment valve and also suggested flushing the unit.

Open to any ideas/insight - thanks!