Hello I'm writing to you regarding my wife's 03" ford explorer. When she goes to turn the heat on she told me that when she turns the control knob it makes a clicking noise it used to click for a few seconds now it clicks till u turn the knob to the off position. She says the heating vents won't open up allowing the heat to come out. My? Is what's causing the clicking noise when the control knob is turned on heat and why is the vents not opening like they should? Any advise would be greatly appreiciated.