The ornamental design for a knob rattle, as shown and described.
FIG. 1 is a top perspective view of the rattle;
FIG. 2 is a bottom perspective view of the rattle;
FIG. 3 is a top plan view of the rattle;
FIG. 4 is a bottom view of the rattle;
FIG. 5 is a front elevational view of the rattle, the front elevational view of the rattle being a mirror image of a rear elevational view of the rattle; and,
FIG. 6 is a first side elevational view of the rattle, the first side elevational view of the rattle being a mirror image of a second side elevational view of the rattle.