About a month ago I was returning from vacation in the mountains. About half way home while driving on the Interstate, I heard a fairly loud bang that came from the passenger side of the engine compartment. At first I thought I had run over something and didn't see anything. I thought that if it were something serious I would see a message on the dash. I drove the rest of the way home (about 200 miles) and no message. About 3 days after the trip I started the car and a message appeared stating the coolant level was low and not to start engine. I checked the reservoir and it was empty. I added enough water to it to bring it up to the filled mark. I have driven it now for about 3 weeks and today the message has come back on. I looked at the reservoir again and it was about an inch below the fill line. I don't see any signs of a leak and don't smell any antifreeze? Any suggestions??