Hi everyone!
I have a 2003 FX4 XLT with 110,000 miles, 5.4, I;ve owned this truck for 15 years and absolutely love it! I do 99% of all the work on it myself and it has been meticulously maintained! This evening, after pulling out of the driveway, my wife reaches over to turn on the heat and we hear a "clap, clap, clap" type of sound coming from behind the glove compartment, I'm guessing it's probably something to do with the door that controls the flow between defrost and heat?

 

So...did the system perform correctly despite the noise?

 

That noise is most likely a broken gear inside the blend door actuator or one of the actuators

 

There is almost no chance a 10G F150 had a cabin air filter installed (dealers rarely had the kits to install them) and I bet OP doesn’t even know one can be installed.

 

Update, I removed the actuator and then operated it , holding it in my hand. It appeared to function fine, although there was no "load" on it. I then cut into my heater box and found that my original blend door is still in place, where it's supposed to be and it moves freely. What I did notice though, was some black plastic particles, kinda like grinding dust, on top of the actuator, around the base of the rotating shaft. I'm thinking, possible inside of the door coupler that slips over the actuator shaft has stripped?

 

That happens a lot, to see some plastic on top or the actuator
Take the actuator box apart and inspect the gears inside
If the blend door moves back and forth okay it still might be broken (broken shaft is real common)
I sometimes get those actuators from the junkyard just for the gears inside