Speedy Jim wrote:

Sounds like a hole has burnt thru the heat exchanger, allowing
the (main) blower to force air back into the combustion area.
Better crack open the piggy bank...

You =are= speedy! (I don't think my post was up more than 5 minutes
before you replied.)

Hole in the heat exchanger makes sense. Would this be DIY fixable, a
pro-only job, or are we talking about a whole new furnace?

And now for the dumbest question of all: What would happen if I
blocked off the problem burner (at the jet) and ran on 3 out of 4?

Thanks,
Heatless