Yeah i tought it was the lifters to so would i just replace them with new ones??? just the lifter because it started last weekthat noise is the hydraulic lifters in the engine. the 3.1 and 3.4 V6 were plagued with this flaw, usually starts around 60,000 miles and without being serviced regularly it will get worse quickly. my grand am gt does that same noise when its freezing cold outside and also when its warm, just goes away faster when warmer outside. i use valvoline dura-blend oil in my grand am and it seems to help the lifters prime faster. the only reason the lifters do this noise is because they loose prime overnight and cant cushion the push rods properly at start up and in turn wears out the valve train faster than normal. my grand am has been doing it for years, its a 200-300 dollar fix if you do it yourself. unless you have to replace other parts that are worn out other than the lifters.
as bad as that sounds i wouldnt even bother adjusting the valves it wont help,hopefully your rocker arms are not worn out but i bet the push rods are.