First depending on the type of equipment by 20yrs old most HVAC equipment is on its last leg of life anyway. All the things the people are indicating, silencers, baffles, etc. increase the pressure drop of the system and will thus reduce its performance. If all of the other items are not there...
Armaflex makes a closed cell insulation that will not absorb water or moisture. It has some sound attenuating charagteristics but it is not as good a sound attenuator as fiberglass