Upon playing around a little to get to know the origin of the noise, and looking into the iFixit disassembly reports, I can now describe the problem in more detail.
First, the very high pitched humming noise, does originate from the fan. But this noise alone is not the problem, as it is almost not audible at a normal working distance - its the amplification this noise gets by the hollow case build structure, that becomes a resonating body for it, that makes this worse.
When I lay down the Macbook on a firm, but soft surface, the case part stops resonating, and the level of the humming reduces by about 70%, down to a level, where it isn't such a problem anymore.
I can reproduce the same effect by sitting down the Macbook Air at the upper left corner on my open palm, and then removing it. the character of the sound changes, the humming becomes a deeper and more full bodied.
http://www.ifixit.com/Teardown/MacBook-Air-11-Inch-Model-A1370-Teardown/3745/1
As you can see in the pictures, there is nothing that would stop the vibrations of the fan from setting the whole lower part of the Macbook in motion, as they are fixed only on the edge - Apple created a wonderfull "echo-chamber" for the little, but nevertheless noisy fan, which rotates at 2000 rpm even in idle.
What engineer in their right mind would create something like this? Too much confidence in the vibration resistance of a thin sheet of aluminium?
Again, please let us know, if you are also experiencing this problem - its hard to see this thread getting buried, by all other sorts of nonsense questions, and fanboys acting like here has bee an second coming of some sort...
I find it appalling, that almost everytime the first reaction to a problem here is someone who is trying to convince others, that
a. the problem doesn't exist
b. the problem lies with the individual
c. that it has been a design compromise, because Apple couldn't have done it differently
If this is the "Apple-experience", I know where to look for my next purchases...
