Do remember that they are hydraulic tappets and all engines do give some slight noise at start as the engine gets from the sump to the top of the engine irrespective of being hot or cold though mechanical "gaps" are bigger when cold
If the oil is too thick then the variable timing of the tappets doesn't work
The principle is that as the engine speed increases so does the oil pressure and thus the tappet goes up further and opens the valve further
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