I doubt anything anyone did created the whine, it is common in the W201 and W124 car diffs, they are the same basically. They will typically wear at a certain spot in the gears that creates a whine at some speeds and not at others. Common problem, only fixed with new ring gear set, expensive. M.B.DOC on the forum rebuilds diffs for a fair price. Contact him, he is on the forum as a moderator all the time. He is in Georgia and a diff ships very nicely in a 5 gal plastic bucket.
There is not so much a "problem" with the gears as there is a wear pattern in the surface of them from use. It is common. It is still safe to drive, just annoying when you are traveling at the speed the diff whines at. It creates a whine at certain speeds and not others. If you can make the whine come and go by speeding up and slowing down you have "gear whine" and bearings wont fix it. The bearings may even be just fine. The only way to rid a diff of the whine is new gears and they are so expensive it is not worth it. Talk to MB DOC. he can tell you the low down on the rear diff issues.
Good luck.
One thing to do for the whine is use a heavy gear oil and place sound proofing and or an old blanket under the rear seat assembly, To mask the sound. A good heavy gear oil is fine for the diff and helps cover the whine.
I had a used limited slip rear diff completely rebuilt with bearings and friction plates and washers and it still has a small whine, better then my original diff, but still there. The parts alone cost over $200 for bearings, cups, plates, washers, seals, etc and I did not buy all OEM MB parts. MB parts would have been 2 or 3X more expensive. Still did not buy gears, they cost $850, I would have had $1000 in parts and $500-$600 in labor.
Good luck

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Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red