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 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 ; 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 . - a hybrid power plant provided with at least one combustion engine and with at least one electric machine ; - at least one rotor 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 ; 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 . <EOS>