Soundproofing Metal Ducts attached to Party Wall
I live in a townhouse and I am soundproofing a party wall. The walls are already well-insulated so the only sounds I hear are the neighbors going up and down the steps and some bass music (if the neighbors have the stereo on full blast). I plan on decoupling the wall with RSIC-1 clips and hat channel, puddy pads, 1 lb MLV, fiberglass batt for the air cavity, and 5/8 drywall.
My only issue is that my townhouse has a forced air system with big metal ducts that is attached the party wall, and replacing the entire ventilation system is too costly. I realize quite a bit of sound is coming from the sound vibrations of the metal air ducts.
Is there anything I can line inside of the metal ducts to help dampen the sound vibrations? The metal ducts also have a narrow cavity, so it is difficult in getting more than just my arm through.
Thanks in advance guys for all your help,