You can narrow it down if it is in the clutch or the driveshaft/trans by pushing in the clutch pedal while driving... If the vibration continues after your rpms drop, it is in your driveshaft or your transmission.... If it goes away, it is in your clutch.
I'm not sure what part causes it, but Z33s are notorious for crazy clutch chatter with lightweight flywheels.
Sent from my iPhone using
Tapatalk