In message , Nigel Pendse
writes

What is the motor-type noise that comes from under trains on the Met
line? It's obviously something that spins around but it doesn't seem
to be related to propelling the train?

I think it is the air compressors for the brakes and doors ( I have
read something about this before ).

All Underground trains have air compressers, but they are particularly
noisy on the elderly A stock. I can't remember if they're on all the
cars, or just the trailers.

Compressors are just fitted to trailers on LUL stock.
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