So now we know why the previous tenant had a $2000 gas bill...
The furnace also makes some amazing noises, so we're back to the space heater solution for now. The only real issue with the space heaters is that everything is on the same circuit, so there's a very, very minimal amount of work that can be done when it's cold. You can run a shop vac, but throw in another tool and everything grinds to a halt pretty quickly.
I have also found a tiny piece of history. Joe removed the smoke detector at the bottom of the stairs, and apparently the previous owners didn't bother to remove the smoke detector when they stripped the wallpaper or re-plastered. It almost looks they plastered over the wallpaper, actually. During the summer I found some pieces of plaster in the floor with vintage wallpaper from the bathroom, but for the most part we haven't had many remarkable historical discoveries. There is some wallpaper/brown paper covering in the closets but it's so damaged it's not all that interesting. This looks like a schoolhouse of sorts.