It's the ac compressor. One way to test is to change the remote start settings to use the last setting and not auto. Then turn on the face vents, heat to max, and make sure a/c light is off and front defroster is off. Remote start it and the compressor should stay off and not make the sound. Also, the truck must be cold for the sound, explanation below.
I chased this for awhile and replaced the ac pulley, clutch, and clutch coil. I found that once the compressor sits for a long period of time it gets stubborn to turn and this sound is the first jolt of it turning. I didn't replace the compressor so I'm unsure if a new one will make the sound go away or not and if this sound is a sign of the compressor going bad. However, I remember reading somewhere that a Ford engineer said it's due to the compressor being the lowest part of the system and as it sits all the freon/oil drains down to it and makes it have more stress when it first turns over.