Furnace fan is rattling
These blades typically have a boss that has a set-screw and also might be "keyed" (i.e. has a round hole with a flat spot, to mate with a shaft equipped with a flat area, to provide a landing surface for the set-screw).

Your problem sometimes occurs when the set-screw has come loose due to vibration; check it and tighten if necessary. It is also possible that the boss in the fan blade/propeller was damaged by the spinning shaft, because the set-screw was loose; this normally requires fan blade replacement.