It can be a number of things but to avoid destroying the complete gearbox, it is best to disassemble it and make sure gear movement is not impaired. In my experience [3 gearboxes], I found that the outer casing of the planetary system failed through fatique or stress. The planetary gear had churned the outer gears through imbalance/stress.
That's my 2 euro cents worth.
Stephane, July 2009
