More information: I really only can easily discern the noise when the car is cold after sitting all night. By this I mean - there is an obvious difference in sounds I can hear when I am running the engine in N, and running it in D with my foot on the brake at a stop. But insofar as hearing a loud, pulsating hum, well, that seem to happen only during the first minute or so after starting from stone cold.

I have a theory - maybe it is reasonable? Perhaps this really is as simple as the car generating different vibrational modes when in D and the T/C is stalling due to the car not moving, and I am just hearing that combined with the car being a bit rough due to low temperatures. I know i have a persistent rattle coming from the air cleaner mount (missing a bolt) and if something that small can be so audible, then maybe a worn transmission mount could cause this?

Certainly the car drives well and shifts well - and I ain't no transmission guru but I would think that a serious problem would manifest itself while the car is moving, too...

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1984 300D Turbo, 326000 Km, Thistle Green Metallic