A number of years ago, I had sheered a few teeth in the upper gear of a Bravo III. At the time it was sounding like a coffee grinder. I tore it down to investigate the noise and found the sheered teeth. The process requires that you split the outdrive. I found the same thing. I had also been running just slightly warm at the time, despite new manifolds, risers and impeller.
At the time the B3 was probably 14-ish years old. The case was also showing signs of corrosion, so I just replaced the outdrive.