Driving impressions of my E350 Wagon '05 at 60mph/100kmh. Note the engine revs 1800rpm at that speed in gear 7 which coincidentally is the same rotation a V12 E38 BMW / S600 Mercedes has at that speed.

Video inside my E350 following engine shutoff. The 'mysterious' noise now disappears a few seconds after engine shutoff. I still haven't identified what it was/is. This video is uploaded in preparation

Video inside my E350 shortly after engine shutoff. The electrical/buzzing noise could be heard for an extended time period and originated somewhere from the depths of the dashboard near the center speaker.