Last month I finished rebuilding my rear end on the 300TE

I replaced:
Diff mounts
Props haft bearing, mount and flex disks
New axle shafts

Replaced axle shaft seals on diff
Resealed rear diff cover (insides were squeaky clean)

Subframe mounts
Rear control arms

My question:

My differential has always had a whine that you can only hear when the throttle is at a certain position. It doesn't whine based on speed.

I've always ran 75w90 synthetic in it. Would a thicker fluid help at all or what part of the differential would be the cause of the noise?

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

Diff whine usually means the pinion isn't correctly shimmed. Fixing it requires tearing down the diff.

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)

Mercedes replaced the axle in my 190E 2.6 five-speed twice under warranty due to ring and pinion whine that occurred under "float" conditions such as when you lift to go down a mild downgrade at freeway speed and the input torque equals the drag torque, so there is no load on the gear teeth.

Back then I was traveling between Socal and NoCal several times a year on I-5 and the noise was really annoying.

The axle oil spec for my '88 is SAE 85W-90 GL-5, and I went through hell trying to find any. I finally called Pennzoil that had a product on the approved list and they told me that it was not marketed in North American.

That left 80W-90 GL-5, which is the same as most manufacturers recommend for hypoid axles. It's not "very thick". The absolute viscosity is in the range of SAE 30 to 50 engine oil. Engine oil SAE viscosity ranges from 0 to 60, and gear oils range from 70 to 140, but the absolute viscosity ranges overlap.

As far as ring and pinion whine is considered, changing the drive pinion depth may help, but which way do you go, and setting up an axle is a very precision procedure and the change could make the problem worse.

The best solution on a 25 year old car is probably to turn up the radio.

Ring and pinion whine is usually torque sensitive, so it may only occur under certain driving conditions like what I experienced. A spalled bearing will usually make a constant growl that may be higher pitched as speed increases, but it's not torque senstive.

Duke

The noise comes from the assembly that is worn. take it apart and check it out

The benz gear lube is sold on amazon under the lubro-moly brand.

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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)

Remove rear cover clean and install paste on the gear rotate gear and check for aliegnment pattern. gear position or worn bearings cause noise