The motor of the Nuki Smart Lock operates at a certain volume because it needs power to lock your door. The perceived volume varies based on the material of the door, the type of installation, and the structural conditions on site. Some materials have a strong sound-absorbing effect.
The noise level can also be lowered by reducing the locking turns. To change the locking rotation, simply select the desired Smart Lock in the Nuki app and go to the lock settings. Now choose "Manage". You can then set that your Smart Lock engages the lock with only one turn, thereby reducing the perceived volume, under the option "Locking rotations". This also saves electricity, and the Smart Lock batteries last longer as a result.