This thread, http://www.diychatroom.com/f17/whirring-noise-return-duct-42268/, addressed fixing a whirring noise, which would indicate low return airflow, by a contractor simply installing a vent on the return plenum in a closet. I realize that the OP did not provide all relavant information but this does not sound like a proper way of solving this problem. Do the HVAC pros ever just arbitrarily add a vent to the return plenum to alleviate lack of return airflow?