Did you have any fluid leaking out at all? Usually diff whine is due to internal wear on the gears, allowing for play between them. Low fluid exaggerates the rate of wear significantly.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth