When I got the housing out I examined it closely. I saw no impact marks, and no wear marks on the housing or fan blades. I heard no rubbing and observed no wobbling when rotating either fan blade. I was baffled. Then I looked down into the engine bay to examine the back side of the radiator.
Clearly there was "minor" damage to the fins, but in no discernible pattern that I could detect. Then I saw something laying down near the bottom, and in fact there were two pieces. I didn't take a picture until I had them laying on the table with the fan housing...
These two stips of plastic/rubber were totally chewed up! Here's a closer look.
But it was not obvious to me where they are supposed to be mounted. They are the right length to go along the side of the housing, but... There is no tell-tale sign of something that slipped off the housing (like a dirt line), and they didn't appear to be curved in the way that would follow the outer edge of the fan shrouds.