My YAS-875Ex has been a paragon of stability since I bought it, but yesterday it suddenly started acting up. Almost all tones in the lower octave started up in the upper octave, and only sounded right when concentrating very hard. Tried all the usual things, and several seemed to work, but after two or three tones the "squeak" was back. A leak light just showed that everything was in order, no visible leak anywhere. A close inspection showed that the felt between the body octave key and the "G" lever was worn, so I exchanged it for a piece of cork. No luck.
Then, by chance, as I put the neck back on the sax body for the umpteenth time, I noticed that the neck octave key seemed a bit wobbly. A closer inspection showed that the screw holding the key had worked loose and was close to falling out. After tightening the screw (many, many turns), the neck octave key felt sturdy again. And - the problem had disappeared.