What make your engine knock until it warm up?

Typically vehicles that use hydraulic lifters & have more miles on them, tend to have a tapping sound until the engine is "warmed up." The hydraulic valve train is designed to eliminate the gap at all operating temperatures, though overtime the lifters wear out and don't fill with oil as quickly resulting in the tapping sound you hear until the engine has warmed up, and the lifters have been properly filled and lubricated.