To indoor occupants, ductless A/C systems are similar to central A/C systems for noise, but they're generally quieter than portable or window air conditioners because the noisiest parts of the system (the compressor and fan) are outside. The indoor ductless units may generate some noise as they cool and circulate air, but they can be as quiet as about 20 decibels—like a low hum or woosh—depending on the make and model.   However, the outdoor units tend to have similar noise levels to central air, which can be a little loud if you're sitting outside on a deck or balcony near the unit. The outdoor component of a ductless air conditioner can be as quiet as about 58 – 60 decibels, again depending on the model.   Generally speaking, modern ductless systems can be almost silent inside, especially on low settings.