A/C Blows Through Floor Vents but not Dash Vents

My 1997 TransAm blows A/C only through the floor vents but not through the dashboard vents.

Although probably not related, the blower motor sounds loud like a jet only on full (the 4th notch). It seems to be a buzz or something related to the blower movement. The first three speeds have a normal/low volume.

Thanks in advance for your recommendations!

Sounds like a vaccuum issue. Check the vaccuum hose from the a/c head unit back. My 94 Trans-Am had that and it was a vaccuum reservoir leaking.