Assuming there is no mechanical issues what weight oil are you running? And when was the last time the filter was changed?  Thicker oil or a prtially clogged filter would restrict oil flow but improve as the oil warmed up.

It only takes a few thousands clearance for valves to become noisy. While a properly functioning hydraulic lifter should prevent that unseen rocker or cam wear would not. A valve may have become bent, hanging in its giude or have debris under its seat. Any one of those things would increase clearance.

Does it sound like a single clatter..., or does it sound like all the valves are clattering?  Lastly an exhaust leak can often sound like clatter and as the manifold warms up the metal expands the leak seals (or seals somewhat).