A portable air conditioner can be noisy when in operation. The level of noise that it makes depends on what setting you have it on and how old the unit is. Many of these units are very old and the resulting noise that the make is a function of the machine. Given the age of your air conditioner, the best way to reduce or soundproof the unit is to reduce the vibration made by the air outlets and motor.
Step 1: Place the Unit on a Large Rug
The main cause of noise from a portable air conditioner is internal vibration from the motor. To reduce the level of vibration, place an old rug under the unit or partially wrap it with an old rug as well. Make sure not to cover the blower vents or outlets where air is being expelled.
Step 2: Use Blankets
If you do not have an old rug to use or that is sufficient to block the noise, wrap the portable air conditioner with a series of blankets to reduce the vibration from the motor. Again, take care not to disrupt the airflow of the unit by leaving a space that allows air to come out freely.
Either of these steps may result in reducing the level of noise produced by the portable air conditioner but may not completely soundproof the unit. If an older AC unit is too noisy, replace it with a newer, quieter unit that is adequately soundproofed.