[bcj]

I've heard a "honk" once in a while if I apply too much throttle with the clutch partially disengaged.
I always associated that with the pilot bearing fluttering a bit and try not to do that.
Get the clutch engaged fully before romping on it is my solution.

May be totally unhinged speculation. No promises.

edit: listened to the harmonica tune on the first page.
My noise was a couple octaves lower frequency than on the YouTube clip