3 speed semi auto with reverse engine randomly starting making a knock in every gear and reverse won't engage just acts like a second neutral anyone know what could of happened?

 

Assuming the reverse is in the gearbox rather than rear axle, I would guess at teeth sheared off a gear. Only one way to find out and that is to strip the engine. Inspect all the gearchange linkage as you go, as it is just possible to be something in that but, most likely, broken teeth.