Well, something was making a noise from inside the thing. It sounded like a bird or small animal was trapped inside it. Finally I pulled it out of the window to find several years' worth of crud caked on the condensor; some of that crud was being sucked into the fan and blown around.
If air cannot blow through the condensor, the efficiency of the unit is adversely affected, so I hauled out the shop compressor and a toothbrush and cleaned the thing. I also blew out the evaporator. Electricity in Illinois is more expensive this year, after all.
Big surprise: I could feel the cool air from the thing on the other side of the room after I finished that little task.
This house was retrofitted with central air in the late 1970s. Sometime in the late 1990s, about twenty years after the central air was installed, it quit working. The compressor would run but the air would not get cool. The thing had a couple of years of flaky operation before that, so it was pretty obvious that it was shot. Probably all it really needed was some cleaning and a fresh charge of freon, but because it was made in the 1970s, it uses actual Freon (some flavor of dichlorodiflouromethane, DuPont patent number...). Since manufacture of CFCs is now banned due to overzealous eco-nuts manufacturing a eco-catastrophe from data which was (at the time) thirty years old, guess how much it would cost to get a fresh shot of coolant in the thing? (And it probably requires a lot of it.)
And replacement--with both Mom and Dad retired, Dad was reluctant to spend the thousands it would cost to replace the central air conditioner, because--in all probability--it would mean replacing the furnace as well. And, as it turned out, the room air conditioner he bought instead was actually pretty effective at keeping most of the house cool.
When I moved back here in late 2003, I lasted until July 2004 before getting a room AC for my bedroom, which pretty much takes care of this end of the house--and so the central air has not been, and won't be, repaired.
The shop compressor has paid for itself again, though.