clicking

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...

So I have a Littelfuse 233P-1.5 Pumpsaver that my well contractor installed. My well is a storage-tank design, with a down-hole pump going into a storage tank, then a booster pump and pressure tank going from the storage tank out to the water usage points. The down hole pump is a 3/4 HP Goulds...

I have a Lennox furnace that was installed probably in the early 1980s. It has an intermittent pilot light. Normally when it's trying to ignite, if I'm standing next to the furnace I hear a repetitive clicking sound; several clicks per second. Then I hear a whoosh as the pilot light ignites...

Navien CR-A 240 Tankless Hot Water Heater installed in 2010. Making an unusual clicking sound as heard in linked video. Has anyone heard anything like this? Sound appears to be coming from inlet flow adapter/flow adjustment valve/flow adjustment v/v pipe. Noise begins at 00:15 (sec) in video...