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The thump most people hear is not a blend door (how can a plastic blend door back that kind of sound anyway). This is a myth that has spread faster the the myth of Sasquatch.
The thump most people hear is a slug of refrigerant entering the accumulator (IIRC). This is a different phenomenon than compressor slugging, which is a thump when the compressor first starts turning due to liquid refrigerant collecting in the compressor. There is a TSB about accumulator noise, and a revised accumlator, but its only for earlier modes. I don't think Ford (or its supplier) ever got the system redesigned to eliminate the slugging, since its still a trait of these trucks (on a hot restart). You hear the thump on the firewall because the evaporator and accumulator are mounted to the firewall, and when they vibrate with the slug of refrigerant, it is transmitted throughout the firewall. The intensity varies with the previous off time of the engine and heat soak. I'v even heard it between compressor cycles, when its cool out (50's) but sunny and I have the AC on).
BTW, In order to hear it, the HVAC controls must be in a position to run the AC compressor, which is all settings but floor only, panel vents only, and off.
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So I assume that you highlighted "most" because there are alternative theories. Doesn't matter, the solution is the same, turn it to off if the thump startles or bothers someone. I have been told by Ford Mechanics that the sound is the blender door, never had a reason to question it when someone gives a diagnosis and a working solution. Now I am curious, I think that I will unplug my pump one morning and start up in AC and see if i get that slug knock you are referencing. 106