My bet would be that there is (was) a cavity inside the hoppered coal that collected/accumulated the volatile gases from the hopper coal as it was heated, that would eventually grow large enough to leak into the firebox and be ignited, setting off a mini-explosion. The cavity could have been created by a chance arrangement, inside the hopper, of large-ish pieces of coal, a piece of wood, etc. that were arranged in such a way to prevent the coal above it from flowing down by gravity. This would explain why you only observe this rarely, as well as several other characteristics of what you describe:
The regular/periodic nature of the mini-puffbacks would be expected with volatiles being released at a constant rate (stove temp being more or less constant over short time frames) into a space of fixed/constant volume.
I assume the sound that accompanies each puff is from air flow thru your ashpan vent holes. The sound seems to have two distinct elements--an "inhale" and "exhale" phase, or if you prefer expansion and contraction phases suggesting, to me, these are explosions rather than just transient changes in airflow, in one direction, thru the coalbed (which was my first thought)
When you poked down thru the center of the coalbed to the grates, enough coal fell from the bottom of the hopper to destroy the floor of the cavity so, after you did this, you observed no more puffbacks, since the gas being released from the hopper coal was no longer kept separate from the fire.
As you know, I have the hopperless 983 similar in other ways to your stove, and I've never observed anything like this. I believe the 50-93 has a hopper, possibly explaining why RLB112 has seen it in his Hitzer but I never have. I suspect the presence of a hopper is the critical issue here.
If/when it happens again, removing the hopper lid and poking at the coal in the hopper to be sure none of it is hanging up (and creating space for a gas pocket) should resolve the "problem".
Last edited by Ashcat
on Thu Dec 23, 2010 12:54 am, edited 1 time in total.