The quietest portable units on the market today are the DeLonghi and Royal Sovereign. They are in the 50 to 59 dBa range.
Think about the indoor spaces where the A/C will be installed. Will it be in a closed off area, such as a sun room, which has no HVAC ventilation?
Is the area simply extra space such as a play room or guest room? Will the unit go upstairs where the environment is really warm, especially in those summer months?
Do you need the space to be quiet because you are making phone calls or simply don’t like any HVAC noise nearby?
Even the finest removable units make more noise than central and window air conditioner models.