Under 12db is supposed to be silent but it also depends on whats in the way of the airflow. That is what makes all the noice, the vibrations from airflow, loose screws in the case etc.

Sometimes a 80mm delta can be quieter than a Panaflow at 24db.

Bring a battery, pref. a 7 or 12V to that store and test the fans by connecting the wires to + and - . That way, you can compare what the fans sounds like in the free if you know what i mean.