You may want to google various range hood manufacturers and download their spec sheets. We have a 48 inch range hood over a 48 inch gas range. Noisy? Yes. On medium to high speed. Did we get use to it? Yes.

Ours vents thru the roof. We have a Prestige range hood system with the blower in the hood. I would think that an exterior system would be quieter but am not sure. We have found that after we get good air movement after 10 or 15 minutes, we can lower the fan speed somewhat and maintain good ventilation.

Don't overlook Broan, Prestige, and others; and there are some good deals on ebay from manufacturers authorized dealers.

Best of luck.

Theoretically, more pressure can be generated with a fan than vacuum can be generated. For very restrictive ducting, an internal blower would be better, if designed for that restriction. This is not the case for most residential ducting, and is not the case for the blowers sold for residential use. Pressure drops either way are a tiny fraction of an atmosphere.

Given the same hood and same actual flow and same fan blade tip diameter, then an internal blower will likely be louder than an external blower, which in turn will be louder than the same external blower with an intermediate sound suppressor. Noise comes from the motor, the fan blades, duct flow, and where applicable, the grease separation baffling in the hood. Some noise is inevitable. This is because grease separation requires high velocities somewhere, and high velocities make noise.

Generally, larger flow fans that are run at lower flow rates will be quieter than smaller fans run at their flow rate maxima. Larger flow fans are more easily packaged externally.

Larger systems using 10-inch diameter ducts should be fine with normal house run distances (unless your house is Neuschwanstein castle).

External fan assembly sizes may be found by researching the web sites for Broan, Wolf, ABBAKA, and others. Many of the larger flow rate residential housings that I've noticed are designed to blow air down the roof from relatively low-height box-like designs. Others use up-blast fans like most commercial units.

All protruding objects are visible against a roof, if that part of the roof is visible from the ground. Obtrusiveness is something only you can evaluate.

kas