Slight clunk coming from driveshaft or diff
 

Apologies for the confusing title, and I first want to mention my car drives perfectly fine, no strange noises whatsoever, this is strictly a noise I hear when I make the clutch/flywheel clunk on purpose.

When I let the clutch out fast (at idle)until it grabs to make the clutch/flywheel clunk on purpose I could also hear a slight clunk coming from right behind me. Obviously there is going to be some play in the gears and driveshaft, just curious if it’s normal noise. I only just noticed this noise, but I’m sure it’s been doing it for a while or maybe it’s whole life. Has anyone else noticed this or able to recreate the noise? I have already replaced the diff bushings, so it leads me
To believe it’s a normal noise.

Subframe bushings can make a clunk also.

It could possibly be a faulty u joint on your driveshaft. I had this same issue you are describing and after diagnosis the u joint was loose and had play in it. I just purchased a brand new driveshaft and installed it and the clunk noise went away permanently. In my experience I have seen this being differential or suspension related as well. The clunk noise will just get worse over time. If you can’t diagnose this noise yourself take your Z to a reputable shop or a Nissan dealership service department you trust and have them diagnose it for you good luck.

As old as your Z is, I suspect that the problem is the subframe bushing (as per Spooler).

Could be rear wheel bearing needs adjustment, but that is usually described as a click not a clunk.

Could be a u-joint, but that doesn't seem to be a common problem.