So here is an update, and more info:
I took the car by the mechanic, and he agreed it was a very strange noise . . .
Car has new auburn diff, new ring and pinion (3.55), new springs (mid-eye), new bearings and retainers, new oil seals, and the rest was from before (poly bushings).
Car set up for street/strip
.014" pinion pre load
load side positive
The car makes a *loud* vibration noise (not a whine, it's between a whir and a growl, but loud) on deceleration (engine braking, neutrel coasting nearly silences the vibration).
From the descriptions in the shop manual, I don't have bearing whine or clicking or grinding noises, as it doesn't happen under acceleration, and no chage when switching left and right.
I'm not sure of the brand of the pinion and gear, cost was $170
Things I've done myself:
I tried rotating the driveshaft so that it is 180 from the position I believe I removed it from relative to the pinion (mechanic painted the diff and covered up the scratches i made). I jacked the car and made sure that everything ran smoothly without load, which it did. The noise was muuch less when jacked, mainly because the car's inertia was not working against the engine in a coast. I also jacked the axle up a few inches so that the pinion angle wasn't as much, but no change.
What I noticed:
On accleration, the pinion gets pulled upwards relative to the carrier, *and* the axle housing rotates slightly when the car is jacked up. The whir happens on "coast" when the pinion drops back to the default position. I can see the driveshaft shift in position a little matching the movement of the carrier. THe noise does appear to be coming from the carrier only.
The mechanic told me to bring it in and we'll pull it and look at it.
So anyway, if you recognize what this is, I'd appreciate hearing it, the mechanic seemed puzzled by the noise and couldn't offer a suggestion off-hand.
So, wish me luck, hope the gears are not fried already.