Reducing AC noise

Noise is caused by turbulence. So look for any place that has sharp edges and make them round. Vents are designed carelessly when it comes to noise. If you really want to go after it, any place there is a lip, build up the area behind it with epoxy, so that the duct work tapers into the vent. I have the old conventional roof AC units. I know when I pull the cover off, it is tons quieter. I've been tempted to smooth out the openings and see if that helps. It will probably help the air flow as well.