I tried the first and least intrusive suggestion that I saw. Heated the extruder to 250c and then selected 'load left' to push through the already loaded extruder to get a bunch of filament rolling on through. I'm running a test print now, and the clicking seems to have stopped. I think I'll wind up cleaning it out at some point, but I'm back to being able to run ABS through both extruders reliably.

Thanks to all for the link pointers. I had been rummaging around in the forum, but apparently didn't read far enough. Sorry for any trouble, and the gnome army is grateful.

Cheers,

John