My 96 would do the same, slight knock when i pulled out then go away. The mechanic at the dealer would simulate the knock by holding the brake and putting the engine under load by letting the clutch out part way. The knock would get louder as the engine began to stall. He didn´t know what to make of it so a Yamaha rep came out they concluded it was coming from the water pump gear it was replaced under warranty. After the repair the mechanic could force it to make a knock but I don´t hear anything when I ride now. That was 13 years ago bought new road to work every summer since the bike is a work horse no problems only normal maintenance love it.