*Could be as simple as a pinhole shooting coolant onto a hot manifold when system is under pressure...or the more common destination of missing coolant. Which unfortunately is coolant heaven after its burned in the combustion chamber. If there are no obvious signs of external leakage, and the interior is nice and dry, might try pulling your spark plugs and look for one or more that are "clean" compared to the rest. Head Gasket failure is rarely a quick catastrophic (wow big word for me...) failure. Starts off un-noticed, then the sure signs appear like rough idle or missing coolant. As it progresses it becomes more obvious then it fails and you get the white plume out the tailpipe. Best to check it out sooner than later. Hopefully more of the V6 lovers will chime in....
~Darin <img src="/forums/images/graemlins/baby.gif" alt="" />