GTO Thyristor power control, also known as choppers as they turn on and off rapidly to control power. It's all solid state, no moving parts, but still noisy due to the high frequencies. Quite high tech at the time, as BR were still churning out resistor controlled units for DC and would do for some time (until the 319s in the late 80s). It's the same as the whine you get when a 319 pulls away, or the early AC motored stock. The reason you can only hear it inside is because the equipment is placed in a box in the centre of the car, so it's loud inside, but quieter outside. The noise goes after a certain time when the Thyristors are turned off and conducting fully, then on higher speed sections comes back again as the motor grouping is changed to parralel. On trains you don't tend to hear it because the thyristors are underneath so the sound is quicky drowned out by the motors and track/wind noise.