Don't think it's a power issue at all, but noise floor issue. In the cooler mornings, I open all the windows, and doors. Trying to air out the house to cool it down for the hotter days. With my from door open, I have to turn up my system, with the door closed I turn it down to get the same sound level to my ears. I live on a dead end street, with almost no ambient noise.
In my HT room, the HVAC is on the other side of the wall, with a big air return in the ceiling. In there, the HVAC does affect the sound, but as above, it's a noise floor issue, and not a power issue.
Good call out, with adding damping material to the heater ducts. Think I'll give that a try.