Help: Tang on random strings at random frets

Just switched strings on my electric (have done this dozens of times over the last 10 years at various gauges) with absolutely no problem whatsoever. For some reason, when i play between the 12th-15th on my A, D and B strings, I get a dull twang to the point where you can’t even hear the note.
Once I move outside of those 12-15 frets, it sounds completely fine…
Help!