The noise almost always comes from the converter. In its ink chamber lies a small, loose metal spring, which moves with the pen and makes sure that the ink flows reliably down to the feed instead of clinging to the wall. In a dry pen that has never been filled, it knocks audibly back and forth when shaken. That follows from the way the converter is built; it is not a fault and no reason to send the pen in, let alone to open it up.
Springs of this kind are found in practically every modern converter, other manufacturers' included. Pens with a true piston mechanism have no need of one, since there the piston itself travels through the reservoir; a piston filler accordingly rattles not at all. A rattle therefore says nothing about the quality of the pen, only about which filling system is fitted.