I'm driving into a dead end, if someone could please help me solve a mechanical issue. I own a stock 1972 240Z, built 12/71 (second owner) for 23 years. I have always maintained it well, a very reliable and always an enjoy to drive. The rear diff have always been making clunking sounds for a few years, until now, a new metal rattling sound, almost sound like the spinning of the "Wheel Of Fortune" but more metallic. I've done some research and I figured I might as well replace the diff since I have a spare. I spent a whole day yesterday to remove and replaced the rear diff with new gear oil, also checked all the U-Joints, any obstruction like E-cable line and bushings all good, after a test drive, I noticed the diff was much better than before (one clunk instead of every shift, lol), figured I have solved the problem!  NOT. Then, that metal rattling sound came back (Arrrgghh!!!) only for a few seconds or more on, and off when I pressed the clutch. It doesn't do it all the time, but comes on more when driving under 30 mph but okay to drive on highway without any issues. Anyone out there might have had this experience and solved it? Before looking into dropping the original 4 speed transmission that could be the culprit? 
Thank you in advance.

Throw out bearing?

One hair off a rat's a s s.

Thanks site I will look into that. But the rattle is more coming from the back, could it be just resonating towards the rear end?

Is the sound constant? Does it change depending whether you're going straight ahead or turning? Is it speed related or does change when you change gear?

Can you post a recording? I'd describe a diff with the pinion shaft moving in the housing as sounding like a wheel of fortune. But that won't be constant, and would imply that you've got two dead diffs.

It is not constant. It goes away when I press the clutch and the rattle comes back on going straight or turning even shifting to different gear at low speed, goes away when I accelerate. The first time it rattled for a long time was about 10 seconds going uphill cruising around 30 mph. I checked the new diff before I installed it and it was pretty tight and smooth. I opened the old one yesterday to check for any chipped gears and did not see any damage. I have a few spare diffs, how do I know if they are good or dead without opening it? I will try to record the sound when I drive it today.

exhaust parts, heat shields, hangers?

@240Ziggy Aside from the rapid clicking do you feel any vibration or is the symptom only noise?

I did check underneath again yesterday for any obstruction. I jacked up the rear wheel and spinning it, the only thing I saw the drum brake fins scraping the dust plate. I cleared that and thought that was it. No, it came came back again after driving home.

 I will probably jack up the rear wheels again safely, start the car and put it in gears to see if I can detect the noise. Just a little scared doing this way though. 

4 minutes ago, Mark Maras said:

@240Ziggy Aside from the rapid clicking do you feel any vibration or is the symptom only noise?

No vibrations. Only a semi-loud metal clunk in the diff that I don't like.

I was able to capture the rear diff clunk and that sound of the Wheel Of Fortune. I place my cell phone inside the ashtray just above the trans tunnel. The rear diff clunk starting from 1st gear, then the rattle starts more when the car has been driven for a while, you can hear it as soon as I shift to second, third and a bit on fourth up to 40 mph and goes away. Please let me know any feedback. Thanks.

 

20190627_154403_clunk.m4a 20190627_154403_rattle1.m4a 20190627_154403_rattle2.m4a

None of it sounds like the diff to me - diff clunks and bangs are a deeper note.
This is more like something hitting the exhaust. Maybe.

I agree with jonbill. It doesn't sound like a diff noise.

In your first post you mentioned changing the diff. Did you replace the diff or just the oil in the diff?

If you changed the diff and it is still making this noise, it's not the diff internals.