Why is the bath exhaust fan noisy only when installed in the ceiling box

I think Chris is right, warm moist air from outside is coming into the vent and condensing on the cooler inside surfaces.

Fixing this could be as easy as fixing or replacing the outside vent damper so it closes properly. You could also go for a vent damper that installs within the vent itself. And finally, you can insulate the pipe so that the vent pipe stays close to the temperature of the air flowing through it rather than the inside or outside of the home (depending on where the pipe runs).

Since yours vents into your attic space I would never use it either. Probably the biggest use of a bathroom vent fan is to help keep mold down in bathrooms, especially in humid areas. Lots of lawsuits against housing tracks have happened in humid areas where builders just put the cheapest fan in and a couple years later the residents find mold. Most bathrooms that are bigger than a closet have undersized fans and don't really do anything but make noise.

Building code changes will happen, if not all ready changed, that homes will have to have a certain air exchange per hour. This is because the modern homes are being built pretty much air tight, or about as close to it as you can get with doors, windows and other entry points for air to get in a house. Builders are starting to use bath fans for this, and a lot of them are being designed to be used 24/7, with very little noise.

Either way, do not pump moisture into attic spaces.