I have been holding off on asking this question to the masses as it is a weird one, but I have had the car for 7 years now and its time.......1968 Charger Non A/C.

Basically, I get a bee buzzing noise from my vent/ air box under dash area when that passenger side vent is closed. It starts at a MPH range around 35 and if I crack the vent it goes away. I suspect the air pressure is causing some type of strange internal vibration. The entire heating system functions properly and is in good shape so I thought I would throw this out there before I consider a tear down.

Any one experience this noise from the vent system in the past?

noise inside the box itself ?

sounds like a loose seal on the internal flapper door

I have taken that into account and rebuilt the flapper door assembly. The sound comes from a more internal higher location than the flapper door area. Like just behind the glove box area up in the dash. Are there internal parts of the air box that can vibrate?

I agree it seems to be turbulence related.

There might be something up in the cowl itself. If no one can post a picture I'll drag out a couple I have that are apart and snap a picture or 2 to give you an idea as to what may be the issue .