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Discussion Starter #1
Been chasing a frustrating problem with a 65 GTV running a 4.56 LSD in it. The problem is a low pitched grumbling sound coming from the rear end on decceleration. Upon acceleration the noise goes away completely.

Of course it sounds like the rear end is culprit, the rear end was rebuilt by a local shop a few years back at a significant cost with all new bearings, etc. Despite that fact, the noise persists and I am having to assume they made a mistake in the setup of the ring/pinion. I did notice there is no play at the input flange of the rear differential (seems pinion depth is tight). I have about 4 other rear ends laying around the shop and they all have a little play at the input shaft, which seems proper.

The frustrating part is that I went ahead and pulled the noisy rear end out of the car and swapped in a "low mileage Spider LSD" which I bought from a local dismantler. The noise is significantly quieter but still there. Once again under decceleration only, hum/grumble sound coming from rear end, though quieter than with the previous rear end.

Is it safe to say I had two bad rear ends in a row? I sent out the driveshaft for a re-balance/check just to rule that out though I don't suspect the driveshaft to be the issue, especially since it's been fully rebuilt recently as well.

Oh and lastly, I did all obvious checks, there is no exhaust rubbage on chassis or anything like that.

Thanks BB.
 

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I did notice there is no play at the input flange of the rear differential (seems pinion depth is tight). I have about 4 other rear ends laying around the shop and they all have a little play at the input shaft, which seems proper.
Agreed - quiet differentials do have some play in the input shaft.

Is it safe to say I had two bad rear ends in a row?
Anything is possible, but Alfa differentials tend to be pretty reliable.

I don't suspect the driveshaft to be the issue, especially since it's been fully rebuilt recently as well.
Did "fully rebuilt" include new U-joints?

How is your rear transmission mount? A sagging transmission puts an angular mis-match on the giubo that can cause driveline symptoms.
 

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Discussion Starter #3
I sent out the driveshaft for a re-rebuild and it checked out fine. All U joints are in just about new condition. Transmission mount is also new......
 

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Have you changed the upholstery arrangement in back seat?? If so maybe you are hearing something that was previously sound deadened..

Just fwiw

Ken
 

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Discussion Starter #5
Swapped rear ends yet again and noise completely gone.

I guess improperly set rear ends, and tired used rear ends tend to make noise on deccel......
 

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Discussion Starter #6 (Edited)
Cancel that, noise still present, just under slightly different circumstances. Any decceleration in 3rd or 4th gear produces the sound. Depressing clutch pedal completely eliminates the noise. 1st and 2nd gear decceleration does not produce much noise, but still some. Car is running a light aluminum flywheel from Spruell, wonder if this is a factor.....I think the heavy stock flywheel usually dampens this....
 
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