Cut hole in supply trunk from mechanical room and could not see heat runs, talked with customer about cutting hole in ceiling and putting access panel to cover hole. Cut hole in ceiling to find heat runs were connected. Believe the system was just blowing air past the two runs because fan speed was to high. But fan speed is set to the tonnage of ac and is a variable speed furnace. Installed a turning vain to force air to those two runs. Airflow through the vents is better balanced and customers are happy with results.