Why do older electric trains like the 1972 stock in London have a continuously rising electric noise while newer trains like the 1996 stock sound like it’s shifting gears?

Question

Why do older electric trains like the 1972 stock in London have a continuously rising electric noise while newer trains like the 1996 stock sound like it’s shifting gears?

In: 2

Answer 1

Correction: rising electric noise upon acceleration and braking