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.