I rode home about a week ago, as I was parking, the engine started making a noise like 2 fine tooth gears slipping against each other. The engine shut down, and would not re-start. I assumed it was a timing belt issue. I have the engine out, timing belts are intact, and valve train all seems to do what it should. Does any-one have an idea what may have happened, or where to look before I tear it down?