I would mess with the carb. Believe it or not I messed with the air/fuel mixture screw on my keihin pwk 28. All the way in no noise. One and a half turns out it made the same noise as in your video. Correct me if I'm wrong but worked for me. It seems it was not getting enough lubrication from the gas and oil when mixture screw was out. Possible jetting isue.?.?