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<FONT size=2>Hi I have an audi a6 quattro with the 2.8 engine on cold start up it runs quiet for for about 20 seconds then then goes very noisy which lasts for about 2 minutes sounds like a rattling cat converter I have aready replaced the nearside and have removed the offsidefor inspection nothing broken inside has anybody had a similar problem.</FONT><FONT size=2></FONT></P>


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The rattle could be the Secondary Air Pumps... Did you get this car recently?</P>


My A6 2.7T during cold weather, does this. It's nothing to worry about. It is the emissions cure during cold startup- fuel does not burn well so, they pump more air to allow for more efficient combustion.</P>


However, if the noise does not sound like a small compressor running, youmay have different issue. I would open up the hood and locate the sound... </P>


usually (on my 2.7T anyway), it is located more towards the windshield and top of the engine... black round device...</P>


Again, may not be the case with yours...check it.</P>