It seems when the Silent Wave Motor (SWM) was added the optical design was also changed.
An AF-D lens will be a screw type where the body has to turn the focus via a mating shaft. AF-D lenses tend to be slower particular on the consumer bodies whereas SWM tends to focus quickly on all bodies only marginally faster on the pro end bodies. AF-D are also quite noisy (sound), I guess may alert wildlife of your presence. SWM as the name suggests is virtually silent.
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