Monica, I really feel for you. That kind of noise is just awful. Unfortunately I have no suggestions and I, too, am hoping to learn something. Dadoes- I appreciate your explanation but can anything be done to mitigate the noise?

You need to speak to neighbors (politely.)

The unit could be mounted on rubber damping pads and they can build a screen around it (yes, yes, yes, I know, not too close).

There may be something wrong with it, maybe it can be mounted on a different base. More than likely you will have to learn to live with it. Some things on your side like a fountain or the susurration of bamboo. They won’t eliminate it but will mask it somewhat.

My previous house had a/c added. I wanted it inbetween my house and my neighbors garage, against code. Code required more distance from the neighbors house.

Maybe your community has similar code requirements. A permit was pulled so I couldnt ”cheat”.

I had this problem once. I lived in a row house that was adjacent to another. The two buildings met at the street but there was a narrow corridor between them further back -- lined with windows -- all the way to the rear alley. The neighbor put his AC compressor in that corridor and the noise was unbearable; I couldn't open any windows along the corridor when the compressor was running. His compressor should have been located at the REAR of his row house (i.e., facing the alley) NOT between the buildings.