Engine Cooling System problems of the 2008 Saturn VUE

I have a 2008 Saturn Vue. At 52000 miles my warning light for my temperature overheating came on. My coolant tank was empty. My coolant level sensor did not come on. My temperature light came on only when my tank was empty. I was told by the dealership that my heater core had went out. Because of this coolant leaked under my dash into my car. I was told it had nothing to do with anything I had not done. It just happens. When I was at the dealership a 2007 outlook came in with the same problem. Saturn replaced the heater core at a discount cost to me. I also was having a rattling sound under my hood. No one could tell me what it was. The sound started from the time I bought my Vue in 10/08. It had 22 miles on the Vue at the time. I would call and as if anyone else had this problem and was told no. Each time I would go to take it in to be check, the Vue would not make the sound. I was also told, because it was a new engine it will make sounds until the engine had some miles put on it. When I took it in for the heater core problem, that is when I was told it has the heat shield. It was fixed at a cost to me. My question also is why would it not be covered and why wasn't the heater core cover under the powertrain control system warranty. If the coolant sensor would had came on before the tank was empty, it could have saved my heater core.