under hood driver side door. knocking until engine warms 2-4 min

What is your question?

my daughter has a 2000 gmc sierra 1500 4.8 ltr 4x4 . at first start up engine has a knock sound for 2-3 min then is gone for the day. No check engine light visible doesnt burn oil tune up piugs/wires with in 3 mos. . sounds lower end noise not top end. what do i check and how. is it a knock sensor possibly?