One way to reduce the noise is to add a sound blanket frame around the sides of the unit. We’ve had good luck with the Brinmar SBUHD Universal-Fit Air Conditioner Compressor Sound Blanket Wrap.
This is an effective way to block noise from an outdoor unit when it is constantly running. Install a metal or wooden frame around the air conditioner and fasten the blankets to the frame to dampen the sounds coming from the unit. This type of barrier will block most of the noise generated by the average air conditioning unit. When building the frame, make sure it is between 18 and 24 inches larger than the unit’s housing to ensure it has enough ventilation to work properly.
Another way to eliminate the noise generated by a window air conditioner is to secure it firmly to the window frame so it cannot move. One way to accomplish this task is to place a board underneath the cooler and use brackets to attach the unit to the board. This separates the air cooler from the window so it will not vibrate or rattle against it while the cooler is in use. If the unit is still making noise, create a barrier around the unit and install a sound blanket for additional sound insulation. This will prevent noise from reaching other rooms inside the house.
Another effective way to dampen the noise coming from an outdoor unit is to construct a wall around it that traps the sound and keeps it from carrying to other areas.
We like the Acacia Hardwood Privacy Screen for this.
The wall should be made of wood, which will absorb some of the sound and deflect some of the sound away from the building. The wall should be at least 12 inches taller than the air conditioning unit. This will trap the sound and keep it from escaping.