On my 420 on warmer days (only when the cars is cold), the car whistles for 2-3 minutes and the sound goes away for the day. Tech said nothing serious, just the air filter.

IS this something that I should be concerned and why does it not occur on my 300 or any other Benz? Should I just replace the air filter? Should be cheap, right? The noise is just annoying and on some days can be loud.

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'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)

Air filter??

Just how can an air filter make noise? I presume the sound your hearing comes from the idle air flow. (I guess no filter might allow noises to be heard).

If this is noticable you probably are also not idling at the right rpm. Normally the air volume is not enough to be noticable.

I would verify idle speed and would check the condition of the throttle plate (not airflow plate). Excessive carbon can keep the throttle partially open and/or change the volume of air that the idle motor (idle air valve) gives to establish the right idle speed.

Remember the air entering the motor is; all sources plus the idle motor equals 600rpm (or whatever your emissions sticker says). If there is unusually low amounts of air going through the carboned throttle then the airvalve compensates and adds more. This can become a whistle from volume. Just the opposite can also cause a whistle by having too much air flow and the valve closing to a point where the air flowing through the small hole whistles.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

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?

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'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)

I don't know why your whistle happens at a particular time. I don't because you didn't bring it to me (bg).

If my conclusions are correct, monitoring the system will provide the answers. Is the valve too open or too closed? Why? Does the system do its job? Both during the event and at other times?

This is an "idle control system". Does it?

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician