I agree on looking at the upper intake manifold. They melt inside around the EGR inlet and let coolant fill up the intake manifold. Very common problem on these cars. If you are going to change that, you might want to consider doing the LIM gasket while you have it apart. Also a common problem on the GM V6 engines. I have changed the lim gasket on all the 3 GM V6's we have owned in the last year.