5.3 Tahoe flex knocking in cold weather on start

I recently purchased a 2003 Tahoe w/a 5.3 flex engine and in cold weather it knocks for about 15-20 seconds at startup. I have 2 4.3 chevy's and they never knock. my son has a 2500 Silverado hd and he says his does too ( i don't know what 8 cyl he has)it seems to be intermittent is this common?I just found out that it could be piston slap or maybe the oil filter is allowing oil to drain backwards I doubt the dealer i bought it from went for the best filter any thoughts?
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