Car makes a rumbling sound when turned off?

When the engine is turned off, the heat in it tries to dissipate through its coolant which is not circulating and the coolant in the block may boil a short time. Bubble, bubble. It is not going to blow up, catch fire or turn into a monster. Some cars run their cooling fans after the engine is shut off to remedy this. I don't know if the Neon does. Maybe you have a hard drive uphill to campus and make more heat than a drive on level ground. Maybe you just get off the freeway and park shortly after. Make sure there's the right amount of coolant in the radiator before you start the car in the morning. Other than that, rest easy.