Those magical days when the HVAC is off

I fully understand and appreciate the HVAC sound issue you have raised. Here are some tips I offer, after having 11 new home constructions in Southern California, Northern Virginia, and the South Carolina Low Country. (1) During construction, specify your preferred location for the air handler, intake and output vents, and thermostat. (2) During construction, insist on knowing the wall, floor, and ceiling insulation grade. (3) For a second floor, insist on a sealed wood floor with “glue and screw” versus a nail gun. (4) Investigate carpet padding, material and method of installation before choosing the flooring. (5) In an existing room, ensure that the HVAC access points to the attic or side walls have an insulation blanket affixed to the door/crawl space. (6) Select HVAC filters that balance your need for purified air and the noise generated by air flowing through the intake vent. (7) Have your HVAC system professionally checked for air output flow and balance.  (8) Ensure your windows are well insulated to minimize outside noise and your neighbor’s HVAC systems. (9) Have your roof treated with an environmentally safe sticky substance to deter pigeons and ravens from cooing, squawking, and bouncing off windows and metal exhaust vents. (10) If all else fails, put on your best headphones and enjoy the music.   

I see some of Paul McGowen listening rooms use Mini-split AC units. They are extremely quiet. 

I don't have AC.  Love it.

Increase the # of grilles in your room if possible.  If you have 300 cfm coming out of one grille it is quite a lot louder than 2 grilles at 150 cfm.

Oversize Return air.

Keep dampers adjusted at plenum, not at grilles. 

Excellent variety of approaches - I had assumed someone would recommend a robust power filter or regenerator, or power factor correction, etc. though a few have mentioned getting off the grid via battery power- I assumed I was enjoying less stress/usage on my section of the power grid with my neighbors and myself turning off the A/C for a bit - interesting that the majority see the improvement related to the absence of excess air circulation and the noise from the movement of circulating air and the associated noise created by the various motors, fans, pumps, etc. which makes sense. I figured someone had maybe installed a giant/expensive power conditioner/stabilizer/regenerator to fend off the myriad HVAC noise creating nasties, but not so, not yet.