The Epson R380 started making an odd thwapping noise, which turned out to be the ink tubes from the Continuous Ink Supply System slapping the overly complex interior of the printer. They seemed a bit loose, but it took some searching before I found the top of the clamp that holds them in place:
It’s the white rectangle nestled in front of the ink cartridges, where it fits perfectly into a convenient slot and looks like it grew there.
I briefly considered 3D printing a replacement, but came to my senses:
That should be good until the silicone rubber tubes finally break after a bazillion flex cycles…