Why does my heater make a loud rattling noise when I turn it on?

SOUNDS LIKE THE FAN MOTOR IS GOING OUT OR POSSIBLY SOMETHING IS INSIDE THE BLOW BOX.I HAVE FOUND LEAVES AND PLASTIC BAGS IN THEM B4

Chevy vents are designed to be a perfect mouse house, they always build it near the heater blower and you have to weasel out the blower housing by prying up the unidash under the glove box, after removing the screws (make sure it is in the off position).