Rattle when AC starts - related to fan speed - disappears at higher speed. But possibly related to slow conversion to redirection of air to coming through the Dash ducts. Recently in cold weather (-20 F) there are brief episodes of warm air stopping (air directed to floor and to windshield usually at that outside temp.

The rattle takes place only at low speeds? In all, or one or more mode positions....

Could be a broken squirrel cage, is out of balance(debris), etc.