I was about to post the same question. I'm glad I searched the threads to avoid redundancy. I have a 2010 E63 AMG and still find this very odd and annoying as, at least for my car, it makes that noise after I shut the car off every few minutes for about 30 minutes total or so. The exact noise that comes to mind is the sound you hear when you pull the release valve on a compressor for a quick second. In short, a VERY LOUD quick hissing noise. Now that I think about it. All the MB's I've had have all made some sort of unique noises. On a 2014 E350 when I turn the car off and exit the vehicle at times it makes a strong humming noise from the engine bay and immediately stops as soon as I hit the Lock Doors button on the key fob. On a 2003 CLK55 AMG when I had the air conditioning set on max it would make a fierce noise very similar to the sound of a jet engine increasing power. I always had interesting looks flying in my direction when that happened. Now that noise I thought sounded very cool as it was a mean/fierce noise that gave it a monstrous demeanor. The shop foreman I'm working with informed me that MB is actually working on some sort of quick fix for this. If this "patch" actually comes out and works, for me at least, I'll be sure to thread that intel.