There is no facility to adjust the valve setup as the lifters are maintenance free hydraulic lifters.
It could be that the one or some of the lifters might be sticking but I've never experienced it, if the noise is there on start up and after about a minute clears means the oil is draining out of the lifters when the engine is off. This is common but not detrimental to the running of the engine as long as it clears.
Low oil pressure can also cause the noise to as the lifter oil supply is reduced.
I would take a guess as the noise could also be a rattling timing chain, check adjustment first.
The lifters can be cleaned out but both cams need to be removed, not a hard job and then the lifters washed in a degreasing solvent then recharged with oil before putting back. Time consuming thats all.