Is the Phantom inside my mind?

[USMCTuba]

I recently acquired a YFB822 (thanks Brandon!) and have noticed an odd, sort of "almost" partial between middle C and Db. On both sides of the partial I can use either fingering and get the same sharp C/flat Db without playing the centered pitch. It basically feels like there are two extra notes between C and Db, both of which I can finger 23 or open. I've been playing F horns for four years and haven't had any problems before. Am I doing something wrong? Could it be something in the horn? Is it finally time for me to go into Linguistics?