In message , Jack Taylor
writes
Almost certainly - but you'll only hear them if you're travelling in the
second or third cars of each four-car train. With the lack of motor
noise
from those trailers (power is provided from the driving cars at each
end)
the sound of the compressors is very noticeable.

There are motor non driving cars, or at least there were on "38" and
"62" stock, perhaps someone can let me know if this is still the case?
--
Clive.