The noise is definately the exhaust fan because I removed the blower and now it is sounding like a "normal" microwave should. What I want to know is if there is another way the electronics are cooled. I looked at the GE parts list and can't be 100% sure that the blower I removed didn't also cool the inner parts.

Thanks for the response, but I am afraid I didn't make myself clear. The blower unit is an exterior fan, 1500 cfm, and it is the fan unit that is 30" by 26". I want to mount the fan on the roof. It does have a 4" flange all around it. I want to build a box on the roof that the fan will sit on, hence the plywood box. I need to find out how to weather-proof the box, and seal it where it sits on the roof, and where the fan unit sits on top of it. gammyt - I do not mean a range hood. The range hood will be inside the house, with a 10" duct going through the ceiling and into the fan on the roof. I will be cutting a 10" hole in the roof for the duct. sierraeast - its not a cap, its a large rectangular blower. Luckily my building dept doesn't care about range hood venting systems (except they do care about downdraught systems) Hope this make things a bit more clear

Exposed metal duct in the cabinet can make a lot of noise. You want to prevent vibration (that produces noise). Surrounding it with gooey stuff is a good start. Layers of different materials all help. Surrounding it with a plywood enclosure or chase is good too because it makes the vibrations (noise) get trapped in the plywood instead of traveling through air to your ears. Surrounding it with inert fluffy stuff can help too, inside the plywood enclosure. Getting at least one layer to be truly airtight is an implementation challenge. David

Love your cabinets! There are two types of units: recirculating (mentioned above) which runs the air through a filter. They don't work very well. All exhaust hoods are vented. They have to be. They run the air through a filter, through a duct (or directly through the wall) and outside. That is why you can smell coffee and fried chicken when you walk past a restaurant! The duct can run up through the ceiling or out the back. There are also (less efficient) downdraft models which run out through the floor or wall (down below). Some older homes vented into the attic, but that is generally no longer allowed (imagine your attic full of years of grease). The capacity of the hood is measured in CFM (cubic feet/minute). The more powerful ones also require makeup air - air coming in to replace the air being sucked out. It's not cheap because it needs to be heated/cooled. Some units can be converted from recirculating to exhaust. Where is your kitchen designer? All this should have been dealt with long ago - before the wall was closed in.