The other day while driving with the radio off, I noticed a rattle from the front of the passenger side. At first I though it was a loose mud flap but it continued. Tonight I was wiggling some hoses and wires under the hood and found something I've never seen before (that could be the culprit). Behind the engine on the firewall, there is a plastic flap that just opens and closes via gravity. It sounds like the rattle I hear inside the car but???? The whole assembly can be quickly snapped off of the firewall but then a hole will be exposed to rain leading into the cabin of the car.

The only thing I can think of is that it is some type of air release from the climate control because air can only be push out through the flap and not taken in.

Any ideas?