Last night I bought a 1996 Buick Century V6, from the Toyota dealer . Well I just lived about 4 miles from the dealer and when I got home I heard a noise coming from the engine, I open up the hood and I saw my coolant water boiling, but the weird thing was that my gauge inside wasn't even showing the car overheating, it was about one or two lines below the half way point. What could it be and why?