Diff whine is dependant on speed. It will rise in pitch with increased mph. It will care less what gear your in. The whine will change with mph only, not engine rpm or transmission status. Just trying to help you isolate the issue.

Once I used quality supposedly correct lubricant in mine and it whined after an hour of driving. I changed fluid to Mercedes brand and noise was and still is gone. Turns out M/B product has an additive package to it that other diff fluid not have.

the old OE axle oil that MB use is very thick, more close to 180 cst when cold and has some tough friction modifiers in it - I believe its the same stuff that toyota fill their diffs in the factory with. - I recall it also having some detergents in it to keep the gears clean.

In my case - I use citgo/mystik 80W90 gear lube from oriellys - its shut up a slight coasting whine I was having in my W210. Same in the 124 - quiet and smooth.

recommended by a repair facility that just works on diffs.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)