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high pitch humming sound at startup

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I recently purchased a 1999 996 C2. When I start the car after it has not been driven for a few hours, there is a high pitch humming sound that lasts for about 30 seconds and then shuts off. After browsing the forums for a while, I am under the impression that it is the secondary air pump. Is this how the pump is supposed to sound? It seems rather loud and is kind of annoying. Any suggestions?


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Here is more than you probably want to know:

"Auxiliary Air Pump

To reduce pollutants contained in the exhaust gas during the warm-up phase and to achieve the emission limits of the roll test in the USA, USA vehicles have a secondary air system. For this reason, an electric air pump has been mounted on the left-hand side of the engine compartment. This pump is activated by the DME control unit and blows the additional air through air ducts to the discharge valve. A pneumatic switching valve which is closed when the secondary air system is inactive thus preventing the flow of additional air is installed between the air pump and the discharge valve.


If the engine is started in a temperature range between minus 10.5° C and plus 45° C, the secondary air system is activated for a period of time dependent on the start temperature (min. 50 seconds, max. 179 seconds).

The secondary air system is only activated if the following engine-related operating conditions apply:

• Engine load (TL) between 0.7 ms/rev, and 4.7ms/rev.

• Air mass (Ml) not greater than 300 kg/h

• Engine speed between 680 rpm and 2,800 rpm

• Altitude adaptation factor greater than 0.76 corresponding to less than 2,400 meters above sea level."

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