It is called a coolant elbow they are made of a weak plastic fiber from gm the ones that are aftermarket naps ext. are a good rigid plastic.They are a common problem on these 3.8 there is another one behind the heater core block. It is not that hard if you know how to do them and yes they just push in. If and when you do it its pretty much doing a lower intake job take job and remember to do both elbows never have to worry about them again.