It would depend on the unit, but modern Japanese makes such as Daikin, Mitsubishi and Fujitsu are exceptionally quiet. I installed a Daikin multi-split system 10 years ago. My neighbour was concerned about the potential noise from the outdoor compressor. I assured her that it would not be possible to hear it from next door. On the final day of installation, the units were tested at full capacity. My neighbour craned her head around the wall and asked when it would be turned on. She couldn’t believe it was running full tilt. She couldn’t hear it at all.