Method of optimising the noise generated by a rotorcraft on the ground, said rotorcraft including: - a hybrid power plant provided with at least one combustion engine and with at least one electric machine; - at least one rotor (2,3) driven in rotation by said hybrid power plant; and - at least one electrical energy source electrically powering said at least one electric machine via an electrical network; said method comprising the following steps: - a first monitoring step for monitoring whether said rotorcraft is on the ground by means of a detection device; - a second monitoring step by means of a control unit for monitoring whether said at least one combustion engine is in the started or stopped state and for monitoring its speed; - a third monitoring step by means of a control device for monitoring whether said at least one electric machine is in the started or stopped state, and for monitoring its speed; - a first control step, by means of the control unit, for controlling at least one combustion engine from among said at least one combustion engine in the started state, if said rotorcraft is on the ground, so as to control a speed of said at least one combustion engine until said at least one combustion engine reaches a predetermined speed referred to as a "silent speed", said reduced speed of said combustion engine making it possible to reduce the noise generated by said combustion engine, while said at least one combustion engine delivers mechanical power in order to drive said at least one rotor (2,3); and - a second control step, by means of the control device, for controlling at least one electric machine so that said at least one electric machine delivers mechanical power for driving said at least one rotor (2, 3). - a hybrid power plant provided with at least one combustion engine and with at least one electric machine; - at least one rotor (2,3) driven in rotation by said hybrid power plant; and - at least one electrical energy source electrically powering said at least one electric machine via an electrical network; - a first monitoring step for monitoring whether said rotorcraft is on the ground by means of a detection device; - a second monitoring step by means of a control unit for monitoring whether said at least one combustion engine is in the started or stopped state and for monitoring its speed; - a third monitoring step by means of a control device for monitoring whether said at least one electric machine is in the started or stopped state, and for monitoring its speed; - a first control step, by means of the control unit, for controlling at least one combustion engine from among said at least one combustion engine in the started state, if said rotorcraft is on the ground, so as to control a speed of said at least one combustion engine until said at least one combustion engine reaches a predetermined speed referred to as a "silent speed", said reduced speed of said combustion engine making it possible to reduce the noise generated by said combustion engine, while said at least one combustion engine delivers mechanical power in order to drive said at least one rotor (2,3); and - a second control step, by means of the control device, for controlling at least one electric machine so that said at least one electric machine delivers mechanical power for driving said at least one rotor (2, 3).