When my car sits for at least 4 hours and i turn it on, i can feel a faint irregular vibration in my seat and there is a popping/chugging noise coming from the exhaust which lasts 10 seconds then the rpm's rise slightly and then go back down to a perfect idle and the sounds all stop? Could it be my CAT clearing the carbon out because of some other problem? Its not loud but it just sounds kinda crappy for that ten seconds.

Oh and it doesn't happen when i start the car with a hot engine.

thanks for the help....