The same thing happened to our 7.5 onan a few years ago. Apparently they used inferior epoxy to secure the magnets and they came loose,tearing up almost everything in their path. An onan shop fixed it with all new parts as needed. Fortunately for us onan stepped up and paid about $2600. We were responsible for about $475 above that. The generator had about 150 hours on it and was about 4 years old. When the onan self destructed it also took out the Inverter/Converter/Charger to the tune of $475 (rebuild) plus shipping. I wasn't a happy camper.
Good Luck,
Journey Indiana