A bit of rubble in the grove is my guess, too. Could be 'bout anything. My black tank had a similar problem, that turned out to be the plastic cutout from when they drilled the hole in the top of the tank to install the toilet. It was "found" near the end of a thorough cleaning/flushing, so when I had to reach up the pipe and pull it out, it wasn't entirely nasty.
Rinse the tank well, that might dislodge it. Fill the tank FULL of clean water and do a complete dump. If that doesn't clear it, you, too, might be a candidate for reaching waaaaaaayyy up in there and running a finger around the groove . . . .
