OMG! It says JR on it! Simple as that! ....... No seriously just kidding, I fly 99% JR equipment. I saw the same thing yesterday on a katana at our field. The rudder would look like it is shivering or well gettin cold and shaking when there is no load on it. Didn't do anything in the air because it seemed like as soon as you put your finger on it it would stop. I would also like to know what is causing this.