If your vehicle is showing rattling or vibrating noise from the vents, here are the most likely causes and the parts you'll need to fix each one.
Leaves, twigs, or even small animal nests can bypass the cowl and become trapped in the blower motor's squirrel cage. This causes a severe weight imbalance and loud vibration when the fan is turned on.
If a cabin air filter is installed backward, crumpled, or not seated correctly, it can flutter in the airflow or allow debris to bypass it and hit the blower fan.
The bearings inside the blower motor wear out over time, causing the fan assembly to wobble. This creates a vibrating, rumbling, or rattling noise that increases with fan speed.
The plastic gears inside the blend door actuator (which controls airflow direction and temperature) can strip out. This causes a repetitive clicking or rattling noise from behind the vents, usually when starting the car or changing A/C settings.
Retaining clips holding the internal air ducts or dash panels can break or loosen over time, causing plastic-on-plastic rattling that resonates through the vents.