Check underneath the head where the left skid pad (that the wobble box is mounted to) pivot's off the frame, there is a shaft it pivot's on with a plastic bushing on each of the tube it pivot's inside of. Also there is a 3_8" U bolt on each end of that shaft that holds it in place and they both need to be tight. Another place you could be getting a vibration is in the wobble box, with the arm that drives the sickle straight forward clamp the sickle so it can't move and try to turn the wobble box pulley. You shouldn't be able to move the pulley at all without moving the sickle.