iam a plumber not an ac guy. i installed a ac/heat unit in my attic, the ducting also.what is the best register to use ,that i can balance ,and direct the flow of air. with out hearing that hissing noise.
you would want a wide spacing to cut down on the volocity. check out hart and cooley register A618 or something of this type. more pricer but will do the job and you can adjust air throw 2 ways. I would hope you installed manuel dampers at the supplys so to balance the systems air flow.