An interesting clatter may be audible on earlier model Burgman 650s while at idle (and especially in an enclosed space like a carport or garage).
Many have referred to this noise as a "Diesel - sound"-ing kind of noise... and you are not hearing things because it is true - the early model 650's do that. The noise is due to one or two of the straight-cut final drive gears. These gears spin around on smallish on "axles" - if you will, and use a series of washers/clips/ or some such other devices to keep them "snugged up" and tight in their orbits.
Well - the devices used to mitigate this small free play weren't quite what was needed to totally eliminate the noise - and it is still apparent on later models if you listen REAL HARD while at idle.
This "Diesel Noise" is NORMAL, and there is no need to be concerned about any sort of impending failure.
Again - it is OK - as in "NORMAL!"
Here are some words about this phenomenon.