We had the folks who worked on it last time out last night to fix it again. It turns out the noise was the impeller rubbing on the exhaust blower housing, and it had worn away an arc in the side of it. There were also several melted wires, blackened insulators, and so on.
All of this was caused by the flame being blown back out of the heat exchanger. Guess what that means? Cracked. Guess what that means? Lots of money.
By the time it was all said and done, we decided it was time to replace the furnace. We've got a new high-efficiency Bryant going in this afternoon. Whee. We were persuaded by the variable speed and high efficiency of the system...plus a couple of rebates that make the cost differential over the next lower model reasonable to recoup.
The work won't be finished today, but we'll at least have heat tonight. They worked on it yesterday, but it quit making heat overnight - and it was down to 54 F on the ground floor when we go up this morning. One night of that was quite enough, thank you.