<Update> OK, after about 24 hours with the air intake closed off about 1/2 and with the OAK reconnected, the rumble happens now very rarely. When it does, it's much more subdued and more brief. As far as the glass goes, there seems to be soot forming, about 20% more. It's also black than the brown stuff. I cleaned the unit out yesterday at 4pm and this morning, I though I could see the flame, a portion (less than half) of the glass was covered. I may adjust the air intake blower a bit and see if that helps. Clinker formation is about the same, so perhaps the pile up is due to this batch of Hamer's. They certainly burn hot, but leave a good size clinker pile after 12 hours of operation. It was -5 degrees this morning around 7am. Clear skies overhead and our low, valley location the culprit. My oil buner was engaged last night around midnight for about 45 minutes so not to freeze my baseboard and domestic water pipes. Yesterday's low temps caused my dishwasher's waste line to freeze -- a bit of a headscratcher initially since it was mid-afternoon and plenty warm in the house. But all's good -- for now!