It is most likely your intake manifold gasket. They are known to fail on these Vortec motors. Mine is doing the same thing and, weather permitting, I will be changing the gaskets this weekend. I was planning on doing a write up on it, so watch for that if you want to do it yourself.
also check the heater hoses. i seen them leak at the rear of the engine bay or as stated above----freeze plugs, intake or knock sensor. a good flash light and maybe a mirror to investigate more.....mike
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