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Thanks for the detailed feedback Graham. FWIW, there's been a
discussion on this topic on the Forum. There's at least some
agreement that the 3.45 diffs may be a bit noisier than the 3.91s,
even when set up by a qualified shop (it seems to be hit-or-miss).
Noisy diffs have been a problem for many years; when my dad was a
service rep at Ford they got lots of complaints about noisy diffs in
early Mustangs. My 2008 Mustang has a noisy diff--how's that for
respecting heritage?<br>
<br>
I still have the 3.91 that came out of my BJ8 at about 120K miles.
The wear pattern was barely discernible but flawless. It made no
noise that I ever noticed. I suspect the only advantage to a
heavier fluid is that it muffles the noise a bit more. <br>
<br>
Merry Christmas to all.<br>
<br>
Bob<br>
<br>
<br>
<div class="moz-cite-prefix">On 12/23/2015 7:47 PM, Graham Wilkie
wrote:<br>
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<div dir="ltr">Hi Doug,<br>
<br>
I received several responses; the conventional wisdom was to
remove the differential and have a specialist carry out the
work. That is on my to-do list.<br>
<br>
Until I get to that item on my list, here is what I have
done. I drained the Penrite 80W 90 mineral gear oil, and
replaced it with Penrite 140 Gear Oil - Premium Mineral.
This product claims "for quieting noisy differentials, where
the noise is due to wear, thus prolonging the life of the
unit". <br>
<br>
After about 60 miles of driving, the whine was certainly
still evident. A bit quieter, but not significantly
so. Following suggestions from others, I then added Nulon
G70 to the Penrite 140 Gear Oil in the differential.<br>
<br>
I have covered some 1,123 miles since then. <br>
<br>
<strong>Conclusion:</strong> The pitch level of the diff
whine is lower, more like a humming; it is now easier to
live with at and above 60mph. The most improvement
occurred after adding the Nulon G70. I will continue to run
this combination in the differential for now. Knowing the
result, I would do it again. <br>
<br>
As has been pointed out by others, the real fix is to have
the differential adjusted or repaired by a differential
specialist.</div>
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<div dir="ltr">Hope that answers your question.</div>
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<div dir="ltr">Regards, Graham Wilkie.</div>
<div dir="ltr">Moruya, Australia. <br>
<br>
</div>
<br>
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