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As an owner of a non-NPP car, I don't believe the loud "blatt" sound has anything to do with the valves on your NPP. My car starts the same way, extremely loud and then quiets down after a few seconds. I think this has to do with the engine settings to start cold and warm up the cats quickly. My non-NPP car is so quiet during normal driving but still has the loud start like your NPP.
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