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Knocking when engine is cold

My 2001 GMC Sierra with 5.3engine has a loud knocking sound when started cold! It starts after the engine is running about 10 seconds and lasts for about 30 seconds. I thought it sounded like a heat riser in the exhaust, but understand that they no longer put heat risers in anymore. It’s been doing this for several months with no other problems noted! Am I about to have a serious problem? Truck has 135000 on it & still runs like a dream!

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