Guys - the clattering rattle is when the bike is idling with the clutch disengaged. So it is in neutral, not in gear, and the clutch lever is not pulled in, so the clutch is disengaged.
Actually, the clutch
is engaged when the handlebar clutch lever is
not pulled in. Its friction disc is engaged with the rotating clutch housing via the pressure plate and transferring that rotation to the transmission's input shaft while the transmission is in neutral. Gear shifting will be difficult or impossible until you pull the handlebar lever in to
disengage the clutch friction disc from the housing and thereby cause the housing to cease rotating the input shaft so you can shift into gear. That is why a diet rich in spinach aids quiet idling.
The clutch lever is released, the clutch disc engages with the housing, the housing rotation transfers engine power by means of the friction disc hub's connection to the transmission input shaft, the transmission transfers power through its gears into its output shaft which is connected to the driveshaft, the driveshaft transmits power into the rear drive input shaft, the rear drive converts the power into wheel rotation which results in your moving forward until you dump the moto because you were distracted by the impending choice between having pork tenderloin or tofu scrambled with cumin, peppers and baby kale for lunch and were leaning toward the tenderloin.
Regardless, without hearing the noise, who can be certain of its source? Even when it is heard, certainty is elusive. Odds are large that the noise will haunt you until you accept it—like I've done with the aid of ear plugs, a well-developed lever grip and a determined state of denial—or fix it.