mostly u r gears are coming in between in the mid. Like no neutral position so the gears are remain half engage, That means there must be problem with START plate which is act ually a gear selector. the 1 and 2 has one gap which neutral. The selector is not positioning itself in neutral so either it is coming in between 1 and neutral or 2nd and neutral. Switch off the bike, then find the neutral first then start the bike if the sound disappears then i m sure the neutral notch has issue. let me know...