Transmission Noise

[jpc3006]

The transmission in my 1973 240z just started making a fairly loud noise when decelerating in gear.  The noise sounds like a slight grinding and vibration can be felt in the shifter.  It is worst in third gear  but there is some noise also in second gear. If I push the shifter forward slightly while in third gear or pull back slightly in second gear (just taking the play out of the shifter) it's goes away. The transmission shifts just fine and there doesn't seem to be any noise in first or fourth gears.  Some background information; the car is nearly stock and has 400 miles on it since an engine rebuild with new clutch, throw-out bearing, etc.  The transmission is the original 4 speed which I rebuilt maybe 15 or 20 years ago.  I installed a new oil seal on the output shaft and replaced the conventional Pennzoil gear lube with Redline MT90 before reinstalling the engine. What has me baffled is that for the first 350 miles after the engine rebuild the transmission worked perfectly. The only thing that has changed in the last 50 miles is the ambient temperature which was in the mid-80s and now is a more seasonable 50 to 60 degrees.

I've checked the shifter bushings and replaced the shifter detent balls and springs. The transmission isn't leaking and the gear lube is level with the fill hole. My next test will be to take the console out and remove the rubber boot between the shifter and the body just to eliminate any possible interference from the boot but I do not think that this is the problem.  Is it possible that the shift rods are worn where the ball detent sits or is the transmission just worn out? Any suggestions would be appreciated before I pull the tranny and tear it apart.

Thanks,

John 

[Dave WM]

did you change shift knobs?

[jpc3006]

The shift knob is not the original but it has not been changed since the car was painted and the interior redone last year. I still have the original knob so I'll put it on to see if it makes a difference.

[rcb280z]

I had a similar issue a few months ago. Was convinced to remove my shifter knob and I'm glad I did. It was the culprit. Hope its that easy for you now that you have done all that chasing.

[jpc3006]

Dave WM and rcb280z  - you guys where right on this problem!  I removed the aftermarket shift knob, reinstalled the OEM knob and took the Z on a run this morning. The result was no transmission noise at all!  

I thank you both for the advice as the noise was really annoying and I was about to drop the transmission. 

[Dave WM]

It seems to have happened enough to warrant a sticky note!

[rcb280z]

Good deal. Mine is still quiet after re-installing my OE knob.

[rcb280z]

I agree Dave.

[grannyknot]

6 hours ago, Dave WM said:

It seems to have happened enough to warrant a sticky note!

I'm pretty sure I found the cause of that rattle back in 2014, post #6,  I chased that sound for a year and a half and even rebuilt the tranny and didn't clue in until I removed the shifter knob.

 

[rcb280z]

And if I remember correctly it was you and CO that told me to check my shift knob when my 280 was doing the same thing. 

[siteunseen]

Yes, sad to say but it was grannyknot.  Simply amazing.  I remember teflon tape on the threads but that didn't work.

[zKars]

I remember sharing this same issue and solution a couple of years ago. It has to do with harmonics of the shift lever based on condition of the bushings and the weight of the shift knob apparently. Being on the end of a long shaft, small changes make big differences.