fantech.net has inline fans. So does Broan.

I installed an external 1500 cfm roof mounted unit for our Wolf Pro Island hood. I hooked it to a 3 ft long Fantech silencer in the attic. I can't say how well it works as the project has not been completed yet, but in general an external or in-line unit should be less noisy, and in our case, with the silencer, significantly so. There will still be noise from the airflow, but I expect fan noise to be minimal.

FanTech has very good inline fans made in Germany and backed by a 5 year warranty see fantech.net. 2 years ago I put a Fantech inline fans in my atic venting my MB and its working fine (I think). They also sell an external fan the RVF that I will have in my kitchen soon, that reduces noise.

American Aldes sells the same fans with a lesser 3 yr warranty, but they have have some more useful information on their site then fantech.
At this page
http://www.americanaldes.com/Editable/Insert-products-04_Kitchen_Range%20Ventilation.htm
click on the "Kitchen Ventilation Options (Diagram)" to see how the various parts fit together. They make a $100 sound absorber they say works well.

I bought the 12x12 ceiling register box, turns out it was 15x15 which does not fit between 16" joists without some chiseling. They make a grill with a grease filter that works well in a kitchen. I hooked it up to an exterior RVF 6. Just got mostly installed today so I don't know how well it works.

I bought at iqsource . com and was pleased enough that I would order there again. Many of the Aldes parts are special order and you have to call to get them added, plus take longer to get.

One thing about the Broan wall mounted fans is they all take a 10" duct, that is a big hole to cut in a rim joist that would not pass code for me. The Fantech RVF external fans come in 3 sizes: 4", 6" or 8".

Has anyone actually switched from an internal to an external fan in the same kitchen? I'm curious whether it is worth the effort to install the external fan.

The kitchen guy at HD told me that this type of fan was to remove heat rather than odors and smoke -- That doesn't sound quite right to me.