The DOHC has hydraulic lifters which are self adjusting, in fact there is no way to adjust the valves short of rebuilding the top end! The hydraulic adjuster may be gummed up, try some seafoam or other quality cleaner in the oil, GM makes a good top end cleaner I just can't remember the name right now!

It may not even be the lifters, I had the same sound, and it took the dealership a month to figure out what was causing it, turned out to be a intake valve that was not completely closing on it's own and then on the compression stroke it would snap closed causing the tick! If it hadn't been a warenty job it would have cost me well over $3K as they had completly rebuild the top end, new valves and re-grind the seats etc!.

Another possible cause could be a pin-hole or crack in the exhaust manifold down pipe, an exhaust leak is commonly mis diganosed as valve chatter.