We added a new air duct to our living area that is very close to the plenum. It is about 4 or 5 feet in length. Now when the furnace starts blowing (both heat and cold air are driven by furnace 2 stage blower) it is extremely noisy. It feels a train is rolling nearby.

Any suggestions on what to look for & how to make it quiet like the remaining duct openings in our home? I would like to findout before calling back the contractor.

Thank you.