I would like to make a change to my AC units so they are not so noisey. I have heard that the noise comes from obstructions in the air flow. I have also heard of people redoing the ceiling vents so ...
I did a quick test this weekend. I took off the plastic vent cover on the ceiling and press a piece of foam over the outflow of air. This forced all the air through the ceiling vents. It cut the noise at least 50%, it was amazing.
Now I need to figure out a way of creating a decorative cover as fpresto mentions above and installing it into the ceiling with a large piece of foam to absorb the noise.