I have an issue with heat automatically changing from duct to defrost vents.

What is your question?

I will turn the heat on to allow warm are to flow through the ducts. At some point in time there will be a "switching" noise and it will automatically change from going through the ducts to going to the defrost vents while the button on the controls will be on the for the ducts only. Again some time will pass and the "switching noise" will go from the defrost vents back to the origional position through the ducts. I have replaced the heating/cooling controls twice with the same results. If I play blower motor controls for about a mile (rapidly changing from high to low then high then low) I can get it back to flowing from the ducts and not the defrost vents. Again when I press the button for heat to travel through the Ducts at some point the heat will automaticlly change from the ducts to the defrost vents without the defrost button not being touched. Thanks