It could be a furnace marble.
One that rolled down the ductwork and into the houses furnace. It got a bit to close to the burner and Voilà.
The furnace repair dudes use to find these more often in days gone by. The holes in the grills were much bigger then. All sorts of toys met their demise in this way. One of my jobs as a kid was vacuuming the dust out of the ductwork, I would suck up marbles all of the time, coins always made a real cool sound in the Kirby’s vacuum hose.
I can hear it now—a voice from the past.
“FIRE” QUIT PUTTING MARBLES IN THE COLD AIR EXCHANGE” (little rolling sounds in the background, plunk, roll, roll, roll, plunk, roll, roll, roll.)
Marble—On!!