Rotary Mower- Noises In The Motor?

I have a rotary mower and all of a sudden you hear a rattling sound in the engine and mower stop If you unplug the power and turn the blades of the cycle, even a small path and run it will OK GO for a while then the same thing happens. I watched carefully where something that hinders the blade and I can not find anything. Could it be the engine is endangered? Thank you for watching this. for me