A cold start usually involves more gasoline being injected and additional air via the idle control. In your case, I would suspect that one of the vacuum lines is cracked and allowing too much air to get into the manifold. The O2 sensors will then even out the flow of gasoline and the revs come down to normal.
The "clatter" you hear are the hydraulic lifters being put to work before the oil pressure can pump them up. So long as their noise goes away, all is well.

Klaus
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The more I learn, the more I forget. So why learn?