Need super quiet exhaust fan

Essentialaudio's details/tips are all good ones. You shouldn't need balancing dampers** unless you are exhausting more than one area, or have more than one return grill; in which case balance is achieved by adjusting the size(s) of the different intake registers relative to one another.

** Most externally mounted fan units are supplied with backflow dampers which close when the fan is off (to prevent cold air from entering ;-)

By no damper I was referring to the damper behind the grille. If present it should be fully open, not used to adjust the airflow quantity, and if possible it should be removed altogether. Correct, most domestic fans have backdraft dampers built in at the discharge.

I have 2 remote fans for my equipment closet. 1 for makeup air & 1 for exhaust air. If you want to move air correctly you must use both or have openings to get or remove the air. If you use only one and you have openings it is best to exhaust the air from the top for circulation.