Indeed; 319s and 456s use GTO (Gate Turn Off) Choppers for traction motor control, while 314s, 315s, 317s, 318s, and all of Classes 320-322 (as built) have Thyristor control. 455s have more in common with 313s in this field, utilising camshaft control.
That makes more sense. The 319s are the only MKIII class EMU that runs under both ac and DC. The DC-only 455s* and 456s have conventional EE507 motors/camshaft controllers and resistance banks as a traction system, (as had been standard SR and BR practice since 1935)! The ac-only MKIII EMUs (317, 318,320,321 & 322) had a thyristor control because they only ever ran on ac from the transformer. The 319s have a 750VDC bus which is either rectified ac from the transformer or DC direct from the 3rd rail collector shoes. In an earlier age, (MKI days?) they could then have a camshaft and resistor bank control, but as energy conservation and more performance was required in the late '80s, the GTO chopper enabled a more efficient control to be used. This is why they have been chosen for the diesel conversion.
The 300Hz sound comes from the magnetic components in the traction control - including the motor windings, when they carry high current pulses at six times the ac frequency (three positive and three negative), 50 x 2 x 3 Hz.
* the 455/9s have been modified and fitted with modern Vossloh 3phase motors and system.