Have been trying to isolate the source of a whine or howl I get on my '85 300d at slow to moderate speeds. One day it sounds to me like the driveshaft carrier bearing... next the diff... or maybe even something in the transmission.

Speed dependent, rises from as stop then seems to disappear (or just ceases to be audible) once you're over 40 mph.

Car only has 105k miles so I am hoping this is on the simple side. Started suddenly after a severe one-day 20 degree temperature rise-then-drop a few weeks ago.

One clue: it is extremely temperature-dependent. Seems to be gone above 8C (46 F). Much louder at 0 C (32 F) or below.

Trans and diff fluid levels are fine.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

If its temp dependent then I would be looking at the diff or the tranny and if you say the trans oil is good and doesn't smell burnt then I would focus on the diff.
Have checked the oil in the diff? Its possible that you have a bad bearing in the diff or the tranny.

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'85 F250 6.9L diesel 2wd XL XC LB Centurion, Gone
'95 Mercedes E 300 Diesel W124.131 with a OM606.910 and just over 200k on the clock, needs top end engine work and a transmission.
2000 S500 W220 with a 5.0L V8 gas power.

If you jack your rear Wheels off of the Ground safely and with enough room to get under the Car you can use a Mechanics Stethoscope or a stick or something to listen to the Driveshaft Support Bearing the Drive Shaft is turnning.

I did that on My Volvo and besides the listening the thing was bobbing about; turned out the Bearing Shield was no good and it actually had Sand in it.

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84 300D, 82 Volvo 244Gl Diesel