"Nigel Pendse" writes:

I meant Tube in the specific sense of LU's trains for deep bore tunnels. I
think sub-surface LU stock (A, C or D stock) do make a different sound,
presumably because they're a bit heavier and also move differently on their
(softer?) suspensions. The firmer suspensions of Tube stocks probably excite
different resonances.

It is a long time since I was on that stretch of Jubilee Line, so you
might be right. District line trains make roughly the same noise as
Picadilly approaching the station though. OTOH, I can definitely hear
the difference between District line trains and North London line
trains at Gunnersbury.