This is some what normal, as long as it just once that you hear the noise (ie. you turn the truck on and thump, turn the truck of and thump) the doors operate off of vacuum lines in the engine. When you shut the engine off the vacuum goes away and *thump* the blend door snaps back to its normal resting position. then when you turn the truck on the vacuum then comes back and *thump* the door snaps back to its adjusted place.
If you feel that this is not what is going on, this diagnostic technique can help eliminate this condition
set the blend door to the hot side and start the truck and it should thump when the engine is on or off.
if you didn't get a thumping noise that is good it just means that the blend door is set to the hot side normally open.
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