View attachment 70672
I remember this thread, I have an update for it.
One of my Z's had what sounds like the same as yours, you could change the noise by going on, and off, the throttle.
Me being the perfectionist I am took the car into my local specialist to do the diff, new bearings etc..
Saw him today and he showed me the offending bearings.
Should have the car back soon after Xmas so I'll report back then.
But I have heard since that's it's common for them to have a slight whine at 70mph or so.
The car has only done 46,000 miles but the diff had a slight oil weep that the previous owner had ignored.