I've heard a loud chirp - almost a short screech - come from the airbox area 3 or 4 times when shifting into 5th or 6th at max acceleration. Scared hell out of me the first couple of times because I thought it might be an early warning of the engine getting ready to grenade.
With the FCR's the chirp you're hearing is the slides rattleing. The power pulses of the v-twin engine are spaced far enough apart that you can really make out the sound, inline fours don't give you that sound. They say that the "sweet spot" is to have the FCR's opened to the point that the sound is about to go away, best mixture or atomization or something. ;D
Yeah, I can hear a bit of a rattle coming from the carbs and low rpm, like during idle. Gives the bike a nice 'quack' to go with the dry clutch rattle. I've gotten quite a few interested looks and remarks lately. I like it