Actually,after a long running the rpm sligtly increases(at neutral) than before when the bike was started.Therefore u hear an increased engine noise.But if u start the bike next morning or after sometime when the bike cools down,the rpm comes back to its normal value,that is,around 1300-1400 rpm(at neutral).Thus its not anything to worry about because it happens with every other bike.The nob for adjusting the rpm should never be disturbed.