My thoughts exactly. I have no idea why it could be the air filter (implying when air travels through it). If your explanation is correct for my symptoms, why does this only occur on warm mornings? Why does the whistle go away in 2-3 minutes? How come it does not occur in the winter? How come it does not occur when the car is hot?
My common sense is telling me that this whistle should occur all the time, but it does not. Is it possible that the air filter expands and removes the whistle?
Coincidentally, a Mercedes enthusiast heard the noise and asked me what that noise was one morning, when I told him it was the air filter (just to see what he would say) he said oh yeah, that's it. To my amazement, he agreed with my Tech.
Have you had this whistle happen to you with the same symptoms?
'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)