Update: I opened up the side case...not good. Looks like the nut on the counterbalancer backed off and went for a ride, until it got wedged in the gears on the waterpump. I've got a replacement clutch case cover on the way (complete with waterpump gear), so I think it shouldn't be too difficult to repair. Hopefully. Anybody seen anything like this on a WR250? Seems pretty uncommon.