Air Vent whistling
You may have some debris in the vent system.
Like a small leaf or something.
My brothers' car had that, so I would whack the vents up high, and then shut them off, then up high again etc...
This seemed to dislodge whatever was causing it.
Or at least moved it into a new position.
Noise stopped.
Not saying that this is what is causing your problems, but maybe give it a go and see what happens.



