Munchkin 80m (R1) boiler issues (Boston)

Were there any "coffee grounds" inside the heat exchanger?

With that rear target wall out it is a good time to flush SS coils with water & cleaner. You want to be sure the condensate drain is clear & flowing.

The only Munchkin I installed and service is a 199,000 BTUH from 2004. The install manual recommends CLR spray cleaner applied, brushed with nylon brush, (never steel) and rinsed down thru the condensate drain.
What I do is remove the rear wall target ceramic insulator, water will do it no good. I vac out grounds; rinse with garden hose spray, (you need good control on the pistol nozzle as there is a small drain in the bottom) then spray with CLR, brush, rinse.
Then with an old credit card I slip the card between each coil to cut out any crud. The first time I carded the coil I had to cut small notches in the card to "saw" crud out. Keep rinsing this down the drain. I run the "card saw" around between every coil, maybe twice. There are about 8 to 10 supporting tie wires on the back side of the coil that you can't see. Don't try to saw those thru with your card This card cleaning is time consuming on a 199, but yours is smaller.

One of the main problems with this design was blocked drain. The condensate backs up in the bottom of the HEX, the back wall gets wet and will eventually crack or crumble. The front wall gets wet and shorts out the flame sensor. Factory even recommended gouging out a little insulation around FS and spark assembly in the event the front insulation gets damp. I assume you have a bottom drain trap, don't unhook it, but squeeze it to milk out any deposits....before starting major water rinse. I was fortunate to have mine 16" off the floor.

The condensate produced by the boiler must drain down between the bottom section of the coils and leave thru the rubber drain/trap.

Call HTP and ask for the part number for the complete parts kit for the boiler . Everything in one box . Should not take long to get seeing as HTP is in New Bedford .

While you are waiting for parts, I would certainly insure that the coil spaces are clean and the drain is working well. Then install new parts and check operation. All issues I had were caused by plugged/slow drainage which damaged some parts. But the drain was the initial problem for me.

Are you on natural gas or LPG? Has anything at all changed with the gas supply system?

Did the noise start after the blower was changed?

See the post by GW above.

Your cleaning efforts were not in vain, standard required maint for a mod con boiler.

Check the mesh burner screws, often loosen up due to heat. You will know by wiggling the burner cone. Check swirl plate at gas valve, the swirl plate tends to deteriorate, and then perform C02 analysis with combustion analyzer.

Ask him what he knows the figures should be and then ask him to give you a brief explanation of why...they should be....