HVAC supply vent noise: how to eliminate the whistling wind tunnel?
Forget any suggestions for restricting the airflow at the grille.  The proper way to control airflow in ductwork is with an in-line damper which is placed as far up line as possible in the trunk or branch without affecting any duct drops that don't need the airflow restricted.  The reason for this is efficiency and noise.  I'm writing this from my phone, but I'm going to attempt to include photos.