Possibly a mode door actuator reset , but most likely a bad actuator

It's possible that the ventilation system is fine, check your coolant level - radiator and reservoir, if they're o.k. try starting it (from cold) with no A/C, heat or fans going. If you still hear it, it may not be the vent system. Next, start again (from cold) with the heat and fan speed on high, and again, after cooling, with the A/C on high / max. If one of them is much worse, or much quieter, then you've narrowed the search drastically. Also, get someone else to start it while you listen to the car outside (pop the hood, stay clear of moving parts), try to pinpoint the noise using echolocation. the only other thing I would suggest is that you have a mechanic check the pressure of your A/C system, a small leak could lower the pressure enough to make the noise. Hope this helps, good luck.