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Recently I started hearing a strange "plastic-like" knocks right after starting the engine.

EDIT. Here's the video: http://www.youtube.com/watch?v=i4mLn_-nUYA

I also tried it with the engine off (not filmed), and the knocks are still there. Interestingly enough, when I turn the engine on, hear the knock, and turn the engine off, I do hear the knocks and a very faint noise in the background. When the noise stops, the knocks stop as well.

Knocks seem to be coming from the heater but I doubt whether it's truly the heater. The sound goes away within about a minute or so. If the car is warm it's even faster - about 10-15 seconds.

The car - Mercedes C180 Kompressor. Any ideas what it may be?

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    Can you be more specific about the location of the noise. Do you hear it inside the car from the vents, under the dash, near the blower motor, or the actual heater core. Can it be heard from under the hood as well? Does it do it if you turn on the key and don't start the car? Anything make it change, time, engine temperature, outside temperature, Heater or Defroster on or off.
    – Move More Comments Link To Top
    Dec 6, 2012 at 14:27
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    Thanks for the reply, I updated the question and added a video with the noise.
    – andreister
    Dec 6, 2012 at 21:34

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This is highly likely pieces of debris like stems and leaves, nutshells.

In the good old days if we didn't want to drop the blower motor and clean the housing out we would just put up with it until it moved further up the system and away from the happy bouncy toy we call the blower unit.

A lot of newer cars (i don't know about yours) have an access point where a micron filter is installed and you might be able to get a small vacuum hose in there to clean things out.

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