General rattle detection guidelines:
Small end rattle: Usually heard more at light throttle transitions, idle or deceleration. YZ400Fs are noted for this type of noise.
Piston Slap: On a four-stroke, it is power related; that is, the stronger the engine fires, the more piston slap noise is produced. Usually more evident at lower revs, just off idle. KLX300s are famous for their "klack klack klack..." piston slap noise at lower revs.
Timing chain slap: Is often louder when the engine is cold and is not affected by power on/off transitions. It is often loudest at idle and reduces with revs. Worn neglected XR200s can often make this type of noise.
HTH