I had the same problem with a 2003 yz250 of mine. Does it inch forward in neutral?
Can't tell you exactly how i fixed it but i took the clutch basket out, and put it back in re tightened it and whala the noise went away and it stopped acting like it was trying to be put in gear. my thoughts were that the clutch basket was lose but maybe me turning the bike on it's side fixed it ha. good luck an keep us updated