People are assuming here that it is sound traveling up the ducting.
Keep in mind that the ducting through one's house, in behind walls, is held in place with screws, strapping, etc. ANY vibration, whether it be from the HT or from a garbage truck rumbling up your road, can create noise from any and all of the HVAC parts which are not secured 100% and, additionally, connective parts being secured with a dampening material.
e.g.
The metal strapping around ducting would need to have at least some thin rubber or foam material between it and the metal ducting to reduce vibration noise and i highly suspect NO ONE on these forums, except for those presently building a new room, has this on the HVAC through their home.
Most major ducting would be secured with metal screws, metal strapping and tight to a solid 2x4/2x6 joist/stud, etc.
The HT may be the source of heavy vibration, but the ducting immediately around the HT area may not have anything to do with the 'ringing' being heard upstairs. For all you know that could be from loose strapping somewhere along the chain for which insulating around the HT room won't do a darn thing.
