On Thu, 23 Nov 2006 22:30:01 +0000, Steve Fitzgerald wrote:

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.

Which, on the Met, are carriages 2, 3, 6, and 7.