My mechanical room is adjacent to my Ht room and I was wondering if anybody has any ideas on how to reduce the resonating noise transfer coming from the one of my main supply ducts and my return duct. The ducts run just above a door into the HT room (see pic) I am currently in the process of building a room around the furnace and such to keep the noise down and finish up the basement. I can get some acoustical duct liner and clips but that would require removing the ductwork and reinstalling it. Are there any sound damping methods that I can do from the exterior of the ducts? Has anyone tried using a material like dynamat? I have lots left over from my car installation but I don’t want to waste over a $100 of the stuff if it will not work.