Yes it is hung from the roof rafters. I'm fairly certain the noise I'm
hearing is noise created right at the vent from air flowing through. I
removed one of the ones with slant fins, and it went from locally noisy
(a whooshy whistle) to virtually silent in that area.

The room is about 500 sq ft. It has 2 8inch flexible supply lines
terminating in 10" boxes, and 2 8inch flexible return lines also from
10" boxes. There is a lot of air being sucked through (though I
don't think the fan is at max, perhaps medium). Is that sufficient
ducting for a room that size?

-Jim