On the pro it could be many things. What year are you talking about? It could be as simple as the top hat is bad all the way to broken gears or bearings on the gear shafts. Usually when the bart gear starts loosing pieces it does not lock up, but sounds like a rod knock. But yes, best to pull it apart.
My son went over the bars in my front yard and strangely enough the chain wrapped around the front sprocket. I split the motor the next morning. 2 gears fell out!
Good luck. You got any help?