An unmanned aerial vehicle, UAV, comprising: a UAV body; a plurality of rotor units mounted to the UAV body, each of the rotor units including a bladed rotor that rotates to generate thrust; and a control system coupled to each of the rotor units to control rotation of the bladed rotor of each of the rotor units, wherein the control system includes a controller with logic that, when executed by the controller, will cause the UAV to perform operations including: adjusting a phase delay of at least one of the rotor units relative to another of the rotor units, wherein adjusting the phase delay causes a spread in tonal noises generated by the rotor units and comprises offsetting phase delays of the rotor units relative to each other to offset phases of peak amplitudes of the tonal noises generated by different ones of the rotor units from each other, characterized in that offsetting the phase delays of the rotor units relative to each other comprises: determining a rotation period for one or more of the rotor units; dividing the rotation period by a number N of the rotor units to generate a phase delay value; and phase delaying each of the number N of the rotor units by a different integer multiple of the phase delay value. a UAV body; a plurality of rotor units mounted to the UAV body, each of the rotor units including a bladed rotor that rotates to generate thrust; and a control system coupled to each of the rotor units to control rotation of the bladed rotor of each of the rotor units, wherein the control system includes a controller with logic that, when executed by the controller, will cause the UAV to perform operations including: adjusting a phase delay of at least one of the rotor units relative to another of the rotor units, wherein adjusting the phase delay causes a spread in tonal noises generated by the rotor units and comprises offsetting phase delays of the rotor units relative to each other to offset phases of peak amplitudes of the tonal noises generated by different ones of the rotor units from each other, characterized in that offsetting the phase delays of the rotor units relative to each other comprises: determining a rotation period for one or more of the rotor units; dividing the rotation period by a number N of the rotor units to generate a phase delay value; and phase delaying each of the number N of the rotor units by a different integer multiple of the phase delay value. adjusting a phase delay of at least one of the rotor units relative to another of the rotor units, wherein adjusting the phase delay causes a spread in tonal noises generated by the rotor units and comprises offsetting phase delays of the rotor units relative to each other to offset phases of peak amplitudes of the tonal noises generated by different ones of the rotor units from each other, characterized in that offsetting the phase delays of the rotor units relative to each other comprises: determining a rotation period for one or more of the rotor units; dividing the rotation period by a number N of the rotor units to generate a phase delay value; and phase delaying each of the number N of the rotor units by a different integer multiple of the phase delay value.