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If it has an adjusting ring (that you can turn) it probably shouldn't rattle, and might well be defective.
If it has no moving parts, no adjustability, then rattling is almost certainly a sign of a defect. (The exception to this would be if there were a 'washer' ring on the joint where the teleconverter attaches to the camera or where it attaches to the lens... that might rattle when shaken, but normally.
The ultimate test is, to try it out, and move it about in various orientations. If the rattle were ultimately something bad, esp. if it were something wrong with the mounting of the lens in the teleconverter, then that would show up in various orientations of the camera (photographing at an upward angle or downward, versus horizontally oriented camera photography.
(One thing's sure: if the packaging is damaged, at all, or if it was not well packed, don't bother to test it, just complain and return it for an exchange, with the note that it was probably damaged-in-transit.)
Let me know what you think, and I will advise you further as needed.
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