Not sure but you may have the dreaded 3.4L intake manifold leak.
"From the mid-1990s to 2003 these engines are
involved in intake manifold gasket defects which caused coolant
leakage often leading to engine failure. A series of class action lawsuits, implicating
deterioration of the nylon/silicone material of the gasket are pending on
this issue. Tell-tale signs of such eventual related damage are a
white foam that appears on the inside of the oil filler cap and the
gradually increasing loss of antifreeze coolant (due to seepage
into the intake passages which will lead to engine lock up failure
in such cases). External seepage is also found near the valley edge
of the lower intake manifold."
Source: WAPEDIA --http://wapedia.mobi/en/GM_60-Degree_V6_