The rear end of my 1993 300D 2.5 Turbo (225k mi) is making a fairly quiet whining/howling noise at about 40-45 mph. The whining noise is different when the car is accelerating than when it is coasting at these speeds (louder when coasting). This makes me think that the rear differential is wearing out. The thing is, though, that the noise disappears above 45 mph and below 40 mph. Does my diagnosis sound accurate? I tried removing the differential fill plug to see how much oil is in the diff. but the hex socket on the plug is misshapen and I can't get a grip with a 14mm Allen head socket.

Anyone have any advice on the differential and on removing the plug?

--J

Check for play at the driveshaft, if no slop then it is probably fairly normal, will quiet if you replace the mount bushings. Do check/change the fluid, could be your problem.

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Gone to the dark side

- Jeff

"I tried removing the differential fill plug to see how much oil is in the diff. but the hex socket on the plug is misshapen and I can't get a grip with a 14mm Allen head socket."

Yeah,that's gonna be a Bugger!
'Some have used a "Come Along" attached to the Jack to move the Wrench
(AND the Plug)

If (As I understand your description) the internal Hex of the Plug is Messed Up
...See if Sears or one of the "Tool Trucks" have the equivalent of an "Easy Out"
for the internal hex.
I am NOT going to suggest you drain the Diff and braise the 14 mm Allen into
the Messed up hex.(Too much chance of Involuntary Combustible "Incident".)
Although,(God Forbid) you may come to removing the Diff and taking it to
a Machine Shop so They can!

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'84 300SD sold
124.128

Make sure you loosen the fill plug before removing the drain plug!

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BigBen

'98 E300 Turbodiesel 244 000km
RIP '92 300D 2,5 Turbo 632 859,4km due to engine failure