Whistle in dash vent

If the whistle happens when you close the outer vanes you could remove the director piece and then see if it still whistles. If it does then it is air bypassing the mode door. To repair it you start by removing the instrument panel and the dash assembly. Then you discharge the AC, drain the coolant and remove the HVAC unit from the firewall. Now you can split the case to apply new seals to the door.

You might try adjusting the mode cable first. It could be that the door is not closing fully.

Mode Control Cable Adjustment:

Remove the knee bolster.
Remove the mode control cable from the clip at the heater case.
Set the mode dial to defrost.
Hold the door lever fully forward (counterclockwise).
Install the cable to the clip.
Test the cable for smooth operation.
Test that the door opens fully in both directions.
Install the knee bolster.