Are you ducting through the roof or the back wall? It does not make much of a difference if it is done properly!
Through he roof is fairly straight forward your only consideration is that their are different Flashings for tile & non tile roofs!
Do not hesitate to duct of each motor if there are more than one motor.
You will get a higher exhaust /extraction with less noise!
What causes the noise?
Firstly the motor will contribute some noise but not necessarily a lot.
If you are taking 2x 150mm outlets & converting into 1 x 200mm the noise will get louder!
Lastly the more resistance your BBQ Rangehood has the greater the noise ( Baffle Filters)
Remember when ducting through the back wall you may need an elbow to ease the restriction on the airflow.