Basically, the sound is the sloid-state Thyristor-chopper power controllers regulating the current to the motors. The old Bakerloo & Picadilly trains use the old switched-resisors to do the same thing; hence they gove the "clunk-tick-tick-tick" Sound. For a full orchestra of other Siemens Desiro UK Electric Multiple Units sloid-state sounds, search for: "Siemens Desiro UK Electric Multiple Units | Class 350 360 380 444 450 700 707 & 717"

A great collection of clips

The Desiro family of trains might not be the pretty of trains by god they are certainly comfortable trains going

By golly, there are some ugly looking trains... and some of those liveries don't exactly help either ! 😢