Burnham furnace knocking

eam16

I have a burnham gas furnace wth forced hot water baseboard. I have recently noticed that the boiler makes a knocking sound when it is on. What could the problem be. Plumber looked at furnace and it shuts down at 180 and no air in lines.

Bondo

Ayuh,.... You have a Boiler, not a furnace....

While I'm not a HVAC Pro,...
I'd guess there's a build up of crud in the base of the boiler(inside the water jacket, above the firebox)...
That'll cause a hot spot, 'n the knockin' is a result of it....

how

What's the press & temp on the tridicater on the boiler read?
The first thing to check...Expansion tank.
Tap the side of the expansion tank. I/2 should sound solid (filled with water) and 1/2 should sound hollow (filled with air). If it all sounds solid then it's time to change it as the internal bladder has ruptured.
Does your relief vent water during a heating cycle?
Do you hear water running in the baseboards or heating lines?