Dealing with HVAC vent air flow noise?

The problem is you likely have standard registers (diffusers) on the vents. What you want is something called an Opposed Blade Damper (OBD) type diffuser which has a set of airflow control blades just behind the directional vanes. These (vertical blades) rotate in opposite directions to control airflow without causing a wooshing sound. They are about ten times the cost of standard stamped metal (around $40), but not as costly as adding or enlarging the vents (which would help, but may still require OBS's.)
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I've been working on my 2-part AC system for years, succesfully. It takes time, money, and patience. My attic unit's 11-speed blower assembly (that uses 3 of those at any one time) has been running on the 3 lowest speeds for a couple years. I finally had the last ducting project done just this week, and finally I'm ready to increase the speeds used during refrigeration to 3 and 4 from 2 and 3.

Almost all of my now-almost-30-years-old metal ducts are gone, replaced with larger-diameter flex ducting. EVERY time I have work done on the system, I tell the man we're trying to make the system QUIET, so large-diameter ducts that flow highish volumes of air at lower velocities should be used. What I just finished was the musicroom, where we had previously added a 2nd outlet. This week we plugged the original metal-fed outlet and added TWO flex-fed grilles. BINGO!...more air and less noise.

So yes, get someone to look at your system with a goal of reducing air velocity. Good luck. Where are you?
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Nsgarch, thanks for the tip. I'll look into the OBD's immediately. I'd hoped that a change of diffuser would help. That will be less expensive than duct work for sure. Jeffrey, glad to know you're getting good results solving a similar problem. We added a second air return inlet elsewhere in the house and that really knocked down the noise from the intake located too near my listening area. I'm in central Florida. By the way, really impessive DIY speaker project!

A couple things I've done to help in my room:
Added another vent on the line.
Cut the blades out of the vent cover so it's all open except the frame around the hole. Looks a little funky but helps a lot, and I don't even notice it anymore (who looks at the ceiling, right?).
Have two air returns in the room.
Switched to digital amps (Tact and Bel Canto). My previous Pass monos only lasted a year when I moved to NC.

Photon46, I advise against opposed blade dampers located near the outlets. If you are going to install them, put them well upstream of the outlets, because the sound they regenerate has a chance to be reduced before it arrives at the outlet. I agree with Jeffreybehr's suggestion of larger flex ducts, but just make sure they are substantially larger than the existing metal ones, because the pressure drop in flex duct is an order of magnitude greater than in sheet metal duct, which your fan system probably won't be capable of handling which means not enough air will be supplied into the room. If you can only make the duct slightly larger than what exists, consider ducts made of fiberglass duct board which although they won't provide as much sound attenuation as flex ducts, the pressure loss isn't too much more than sheet metal. Slowing the air down is strongly advised, and oversize the outlets a bit. Using an opposed blade damper in the duct well upstream of the outlet instead of closing the damper on the register supplying into the room can help, too.