have you got a gopro or can lend one? place it on the back foot rest and film your chain, when it make the noise tap the camera so you know where to look during playback and see if the chain is "flapping" if not move it around the bike and look for lose faring's radiator mounts or even just a control cable that vibrates at certain engine revs. Just an idea. My self I think it sounds more like pinking, with a bike cylinder it's going to be loud.
