rotor ballance

I have read many times of different guys (or gals) problems with roter ballance problems never had a problem ever. Anyway I finnally finished my camalina so I switched my roter back into high gear and started speeding her up about 7 to 750 vibrating pretty bad took her to 8 or 850 cut my sample and shut her down . the funny thing was when I was cutting the vibration went allmost compleatly away??? I never got into any rocks or anything so I don't think I chipped a bar or anything I am assuming it is just dust dirt either build up or a chunk of dirt build up misssing . It was getting dark and way to green so I din't start really lock at it but what I can't fig out why I loose my vibration when I have crop moving through? thanks 1688 standard roter