A little more information would give folks more substance to chew on and ponder.
How many miles on the engine now, and what has been the oil change history (mileage interval and oil viscosity and brand)?
Has the engine always made this noise?
If not, what was the mileage when it started?
Did it begin suddenly, or build up in volume over time?
Is the noise only present from cold starts and then quit, or does it persist all the time? If it does quit tapping, does it quit fairly soon after startup, or only later, after the engine is at operating temperature?
I have the 6.4 and it doesn't tap much at all, aside from some pronounced clicking on very cold mornings, but only for a few seconds until the lifters pump up. It's been an impressively quiet engine so far for sure, but then we also have a Subaru and they all pretty much sound like a bucketful of nails rolling around when cold, so compared to that just about anything is quiet.