I was driving on a 75 mile drive (freeway) with my A/C on, vent in fresh mode with temp set at around 67 degrees. Half way way into the drive, I suddenly noticed white vapor blowing through the air vents. I'd never seen anything like this! I kept fan going but turned off the A/C and the white vapor subsided. A minute later, I switched the A/C back on for the duration of the trip and no problems. What was this? Anything to worry about?