Considering the circumstances (all of them) I suspect that during the de-railment of the belt, a chunk of material was torn off the ground drive belt (transmission). Now, as the gapped belt section passes each V-groove pulley.......it produces a bang or thud. It is not uncommon for a belt (or pieces of a broken belt) to be thrown from the pulleys of one system..........into the belts of another system, causing a cascade effect of damaged parts in the other system. The fact that the noise persists even when the deck is not engaged is what sort of points to a chunk of material missing from the "V surface" of the transmission belt.

Look for robins. I used to work nights and walked home from the bus stop. In the darkest areas the dd things would sit on the ground and make noises. After stumbling over several I did learn to shuffle my feet. First time I kicked one by accident it flew up the front of my leg. I am surprised my heart didn't stop beating I was so scared. I did look for any hurt bird after it became daylight but did not find any.

I can't blame a layman or old timer for confusing the terms when in fact years ago there were hot water furnaces. And in fact still are outdoor open system wood boilers/furnaces. You see the term furnace also depicted an open hot water heating vessel. The proper term boiler became prevalent when describing a closed system vessel. So going back to the 50's and before, when risers and open tanks were used in attics, sometimes with feeders, in fact they were "hot water furnaces" judiec, Any fix yet? Being related to an appliance with a solenoid that uses water, I'm betting on a singing PRV on the boiler or it's the tank water heater, or piping.

" even a gentle knock on the wall next to the fixture causes it to start and stop. " Cheap bulbs or loose fixture mounting screws. Not just the screws you may be able to see, but any screws concealed behind the fixture holding mounting brackets to the wall or the junction box behind the fixture. Try just gently pushing on the fixture between the bulbs forsing it tighter to the wall. Sometimes just touching it (even pushing on it or squeezing firmly with your hand) can dampen out the vibration. Remember the bulbs will be flesh burning hot (and any metal near them).