A common problem with our cars is the Upper Intake Manifold breaking down. There are two coolant ports, a send and a return going through it to the throttle body. These coolant ports have the EGR sandwiched between them, which as you can imagine is incredibly hot. GM, in all their infinite wisdom, decided nylon was a great material and since it eventually gets porous and breaks down, the coolant then leaks into your engine and BAM, eventually it'll lead to hydrolock (though you'll get the typical steam out of the exhaust for a while before usually). The Lower Intake Manifold gaskets also eventually break down, and it'* always advisable to replace them when your upper intake starts to fail with the new revised aluminum-framed revision ones. You'd have to tear off the Lower and Upper intake, get new gaskets for Lower, get a new Upper Intake kit (with revised smaller EGR Stovepipe)...I'd recommend pouring a quart or two of dino oil down and all over the crank with the pan open to clean the motor out of any residual antifreeze and then put back together. This is if this IS your problem, but it'* quite common..infact enough to say it'* not a matter of if but WHEN.