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Cold Start Engine Rattle Noise

77K views 87 replies 26 participants last post by  Hoopair01  
The high revs at cold start is for emissions. CATs are not fully effective until they are up to a high temp (~800 degrees). The engine is set to idle at higher RPMs for a few seconds to get the CATs up to temp quicker, preventing fewer emissions from entering the atmosphere.

This is why this happens on both Gs and Qs.