I had a wierd experience today. I own a 2004 Explorer and while I was driving, a tapping noise started coming from the vent at my feet. The loud tapping stops when I turn the thermostat off of max heat and only occurs when the thermostat is set to high temperature. The noise sounds like something is "tapping" on the inside of the vent. I am very curious about this strange noise and would appreciate any feedback. Thanks.

 

could be a blend door motor slipping