Ok so the car has been overheating for a while now. It will always goes back down to normal within a few seconds. Okay so my gf drove it to work this morning and the thing overheated on her. When we got it home a hissing noise was coming from the rear passenger corner of the engine. It lasts a few minutes when I shut the car off and ceases instantly when I remover the radiator cap. What is it? Also a coolant leak on the same side, pretty sure that's a freeze plug? Any advice is great. Thanks in advance.