2006? E350 was taking off...
 

I was over at my friend's horse stable and a lady got into her E350 that looked to be about an '06 or so. She started it up and I thought it sounded quite similar to my car which surprised me. Then after about 5 seconds this whirring noise starts like a jet helicopter warming up, and the sound got higher and louder and I was looking at the car like WTF? And she is just sitting in there like this is completely normal. What was that sound? I don't think she was revving the engine.

it's the ac going into max cool mode. These cars have an enormous electric radiator fan. When it spools up, it makes a pretty good amount of noise. My 2006 does it, and so did a c class service loaner I had the other week.