This suggestion may sound a little off the wall, but it is still a possibility. Check both driveshafts for the header. Make sure that they are phased so that the universal joints are operating in the same plane. Due to the hex shaft, a commone drive between them and two different driveshafts with couplers it is possible to connect the two shafts so they are out of phase. Because these shafts operate at an angle, and if you check the two shafts are at different angles this could be a possiblity, although I think some of the other posts are more likely. How fast is you knife runningIJ Have seen some the can vibrate some at the 600 spm expecially if the header is off the ground.