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Engine noise when idling

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For a few months I noticed the following. When my car is idling after driving for 5to 15 minutes (usually when stopped at a traffic light), I almost always hear a noise coming from the engine bay that sounds like the air pump that runs for a minute when the engine is cold. This only lasts for one or two seconds, so I never had a chance to take a closer look at see if it's actually the air pump or something else. Also, this is happening the first time I stop, but not every time. It doesn't seem related to engine temperature as I saw it happening when the engine is cold as well as beyond 180 degrees. I also noticed that both RPM and voltage go down a little for an instant when this happens. The AC is off so I rule this out.

Does anyone have a clue about why this is happening now? I don't have any fault code, so I suppose this is not a serious issue, but I'd be glad to have an explanation.



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  • 2 weeks later...

This is happening to me as well... 2002 C4S with 45K miles.

The air pump (saw like) sounds comes on when I start the car in the morning, but now is coming on intermittently and much more frequently. I will turn the heater to 60 and see if it still occurs... its starting to concern me.

All thoughts welcome!

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  • Moderators

I have no idea what the outside temperature is in Houston, but from + 4 C° and hotter, the airco compressor is working with the heater ON, if you switch the heater OFF the compressor is OFF as well. There is a chance that you hear the compressor running with too LITTLE R134a ( gas ) in the system, let check the system for a leak and the capacity. ( 850 gramme is needed )

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