While doing a stoppie on my Z250, sometimes just when the rear-end comes up, I hear a clank from the driveshaft(?) Does the shaft adjust or something? Is that a bad noise or normal? does anyone elses Z do that?

 

You mean when you land it does that? Mine does that whenever I jump it and I land with the throttle on. I think it might be the driveshaft and it stops for that split second then goes again. You may be hearing the stop.