An Active Noise Control method is presented for selectively reducing the harmonic content of an ambulance siren. The aim is to implement such a system on the headrest of an ambulance to reduce noise exposure for the driver. Based on the FxNLMS feedforward algorithm, two developments are presented: 1) Reference Synthesis, for those cases where the noise to be cancelled is perfectly known, it is demonstrated that the reference signal can be generated by the cancelling hardware, without the need to employ reference microphones nor connecting to the noise source; 2) Switching FxNLMS, the implementation of separate banks of adaptive filters is presented to individually cancel two alternating components of a bitonal siren, as if they were two independent stationary noises. Demonstrations of the working principles are presented through tests carried out with a synthesised version of the real siren as a noise source, and real-time target hardware mounted on a headrest, in laboratory environment. Results and performance of these combined techniques for cancelling a complex but perfectly known noise signal are presented, and in conclusion further studies, improvements and potential applications are discussed.