From the Wikipedia London Underground 1996 Stock page:
The GTO thyristor used on 1996 stock achieves this by “chopping” the supply voltage in order to drive a sinusoidal current in the motor windings (pulse width modulation), creating the characteristic audible whine associated with the stock and with the Class 465 Networker trains that share its traction drive system. The sound changes as the pulse length changes. The noise is produced by the switching frequency current ripple and the resulting torque pulsation experienced by the rotor of the induction machine.
I’d tell you I’m an amateur train buff otherwise feeling out of his league, but my gyroscopic induction coils fail to provide the adequate capacitance ratio for me to assert I have no engineering training outside core pulsation computing apparatuses of which my home provides power through parallel transwarp modulators.
This experience has allowed me to empathise with some non-IT people who have to talk with me sometimes. When we’re surrounded by computers all day, it can be hard to keep things high level and abstract when talking with people who’s lives don’t also revolve around IT. Or maybe that’s just me.