We first need to establish where the noise is coming from.


If interior:
The blower is behind the evaporator so it's impossible for objects dropped in the dashboard A/C vent make its way back to the blower.

ie.
Vent <-- Airflow ducting (dashboard, windshield, floor, etc.) <-- Evaporator/Heater Core <-- Blower <--- filter <--- Ducting for air suction (recirculation or exterior air)

Some cars may be missing a filter so it's more likely that the object came through the exterior vents.

If exterior:
It may be the radiator fans or the compressor.

It's a pure guessing game for now. Just troubleshoot systematically.