We hear a very loud thumping coming from the wall, where heated water runs through pipes for radiant floor heating. The thumping happens suddenly and intermittently, and is not continuous. Sound recordings: https://vocaroo.com/cyNQ7oq416P, https://vocaroo.com/5yD5hT5qLlm.

Details
* Model: Burnham Series 2PV Gas Boiler. See pictures: https://imgur.com/a/0hyGL21
* Location: Vancouver, BC (Canada). We have a water-based radiant floor heating system. The house is a two-story single family home wood-constructed in 1996. The boiler is from 1996 as well.
* The thumping comes from the wall. Seems to be coming from pipes between the 1st and 2nd floors. We cannot hear the thumping from the boiler room itself. The thumping happens suddenly and intermittently, and is not continuous. We're not sure if the thumping is coming from multiple places, or just a single area and the sound travels.
* The thumping has not affected function. Every room/zone heats up.
* Based on our observations, the thumping starts when the heat comes on.
* We had two different HVAC technicians come by, but the pipes were quiet when they were here, and we didn't have a recording of it yet. I suspected it was water hammering, but the HVAC technician doesn't think so.

Questions
* What is the cause of the loud thumping?
* What are the options to fix it?
* If it is not fixed, how will the condition deteriorate? Can it become a water leak and damage the house?
* Regardless of the root cause of the noise, would you recommend replacing the boiler? It is almost 25 years old.

Thank you!