Varnish and wear inside the mechinism of the hydraulic lash adjusters (lifters) is usually to blame. The parts in the valve-train are supposed to be in contact at all times. Sticking (collapsed) lifters cause gaps which smack as the parts hit. This causes accelerated wear.
Solvents are used to dissolve the sticky varnish. Something like a motor flush can be used, but I prefer Marvel Mystery Oil which will slowly clean the varnish as you drive.
If the lifters are too worn they may need to be replaced, this is not cheap. A rebuilt head may be a better deal.
The PCV system must be in good working order and oil must be changed regularly to keep varnish buildup to a minumum.
