The hook on the end of a tape measure is loose, and a great many people have squeezed the rivets to fix it. It is loose on purpose. It slides by exactly its own thickness: hooked over the edge of a board, it pulls out, so the metal isn’t counted; pushed against a wall, it slides in, so the metal is. Either way the measurement starts from the right place. Tighten it and the tape becomes right one way and wrong the other, by about a sixteenth of an inch, quietly, for good. The rattle was the part doing the arithmetic.