The past week has been pretty hot here in Chicago. My 94 s500 has been keeping me cool, even through Chicago traffic. The engine temperature gauge also indicates the temperature to be at the normal level. A few days ago, I parked the car after a 30 minute car ride, turned it off, and walked out. I could then hear a bubbling/boiling noise coming from the coolant overflow tank.....I think the sound was coming from there. I could also still hear the coolant circulating through the pipes. Is this something to be worried about, it hasnt done it ever since....