97 blazer knocks until warmed up

Every time I fire up my 97 4.3l blazer it knocks and taps super loud until it warms up a little. Every morning I hold my breath. And its only on first start. After it has been run there is no noise. Why is this happening? Is something failing? It has over 100,000 miles

How old is the oil and filter? How is the level?

The oil and filter are fresh I change it every three thousand miles level stays the same doesn't use or burn it between changes...now upon a recommendation from O'Reilly s worker a synthetic oil was used along with mystery oil additive. Was no change in performance with them so have switched back to reg 10/30 oil no additives.

Depends if the noise emanates from the lower end or upper end of the engine. Oil pump failing or valve train, stuck lifter noise.