First, please list year of you car. My 420 makes a whistling noise in the mornings only when the weather is above 60 degrees (from about May through September). The other months it makes no noise in the morning.
When the car is hot (after running a while) or during non-summer months, it make no noise.
I was told that it was the "air filter" by my Tech. I, too, am puzzled on what exactly makes the noise. I was told that it is not harming anything and to just grin and bear it.
'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)