After more "research" (i.e. sleepless nights), it's clear that this noise is the single harness of electrical wires that enters the mast on port side just above the mast step.
Surely this must be somehow secured to the inside of all of your masts to prevent this slapping against the sides at anchor? Or maybe it is somehow fixed in the center of the mast and pulled tight (doesn't seem possible to me though)? How was the mast wiring run out of the factory??
