I went through a similar situation last summer with my GXP. It was indeed a cracked block, luckily for me the day that the milkshake was discovered, was the last day of the 5 year power train warranty. GM completely replaced the motor. They did investigate a blown head gasket and cracked head first, but when those were ok they put a whole new engine in for me. Good Luck maybe it's just the head gasket.