I have a three-year-old self-propelled Lawn Boy that stopped propelling itself yesterday. I think it's the round gear directly under the drive spindle, but I'm not exactly sure. When I put it in gear I can hear something grinding in the gearbox. Has anyone else worn out this gear. If so, do I need a new gearbox or is this piece replaceable. This mower has worked flawlessly for three years and still runs great, so it's definately worth fixing.