I have been having this question ever since I bought my 92 400E, the car drives perfectly, but each time when launch from stand still, I can hear it makes a special sound, it feels like a jet turbine just starts, "humm...", the sound goes way after rpm reaches 2000.

I am not sure it is from engine or transmission, the sound is not loud at all, it sounds not bad either, but I have never heard this kind of sound from any other car I have driven.

400E owners, is this sound normal for your 400Es?

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99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.

if that happens when the engine temp is relatively high, then that could be your engine fan.

V-coupling may be locked in place causing fan whirl- jet like sound
-fad