Various things contribute to the sound, but one of them is how the traction system controls the electric current going through the motors. Due to magnetic effects the current causes some vibration which creates sound.
Classic DC systems as fitted to most of Class 455 switch the current through resistors, so you just hear a constant hum which is the sound related to the "ripple" on the DC supply caused by it being rectified from the AC supply in the substation. They also make clicking noises as the camshaft switches the resistors out as the train accelerates.
The chopper is indeed some traction electronics, not part of the motor itself. It controls the current by electronically switching it off and on very quickly, eliminating the mechanical switches of the older system and avoiding energy loss in the resistors. So you hear a different tone related to the chopper switching frequency. More recent systems use AC motors with the traction electonics producing current produced at variable frequencies related train speed, hence tend to produce rising and falling tones.
