Snapped a pic just for clarity.
The two ducts had quite a few dry oak leaves, the kind that go all crispy and don't disintegrate. There are auxiliary radiators in there, and a shop-vac nozzle cleared them.
The less important, but much uglier one, is the central area, where the same debris just sits and shows. Some of it is visible in the pic, more is scattered across the bottom. I guess they came in via the upper grille. Unfortunately they aren't leaving that way, easily. Maybe I'll just point a stick in there and scramble them to shreds.
