The cheap rear bearing getting worn and allowing the crank to move fwd/rwd and rub the backplate could be what you are talking about?
That is another major factor with these engines.

I have 4 Evo's and have helped numerous beginners w/ various H9 aircraft equipped with Evo's (couple of dozen at least, to answer your (opjose's ) earlier question )
The rear bearings seem to wear and or corrode very quickly/easily. I've replaced many with bearings from either Boca or RC-Bearings.