Steve,
Based on the noise I guess I would look at the alignment
between the two rear u joints and ensure that they have the appropriate
deflection angle between them. I had a car that had a similar noise at
cruising sped that varied with vehicle speed and was a repetitive harmonic.
I cured it by adjusting a combination of the angle of the yoke on the Rear
axle using axle shims and Raising the tail of the trans slightly. I would go
to the driveshaft shop and ask if the deflection angle is the requisite 5
degrees.
As it is under light acceleration (or load) and disappears
off throttle my guess is that you need to adjust the yoke down a few
degrees. Or replace the rear springs as they may have wrapped up a bit
(become slightly "S" shaped) allowing the yoke to ride a little too high
when you are cruising under light load. This is actually bad as the harmonic
may induce a failure of the u-joint at some point. I believe the Pinion
angle should be down about 5 degrees under no load permitting 2-3 degrees of
deflection remaining slightly deflected under load. Weak springs may allow
the yoke to deflect more than that placing it in direct alignment with the
trans and resulting in the harmonic.
My two cents.
Regards,
Tim Ronak
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