I recently installed a zone system for a furnace. The problem I am having is there is a hollowing sound coming from the 2 zones that are closed that the customer can hear in the night.Bypass damper I thought would do the trick. No such luck! The duct work is sized correctly to handle the amount of CFM's. Any ideas. Now I will say the bypass damper fully opened does remove the sound but because there is so much air being dumped back into the return, the limit is tripped. Any suggestions?