A couple of times this week, I've arrived at the Northbound platform at Pimlico to see the red taillights of a departing train disappearing into the tunnel. A short time later (30 seconds?) a very loud sorrowful middle pitched moan resonates from the tunnel for several seconds.
The noise is noticeable from within trains but only if you are listening specifically for it - otherwise it is hidden in the general normal running noises.
metrider, Having watched this video, It appears that the track form and acoustic properties of the tunnel combined with the 2009 Stock reaching it's top speed have produced this "tunnel howl". This used to be a common feature on the Westbound leg of the Jubilee line Extension where trains frequently hit their 50mph top speed as the video below shows between Canary Wharf and Canada Water. As track on the JLE and Victoria are progressively replaced and noise reduction is more aggressively tackled it appears this noise is not as prevalent as its once was. I myself travel most days via the JLE and have defiantly noticed a reduction in "tunnel howl" over the past decade.
Video 1: Victoria line Northbound between Pimlico and Victoria
Video 2: Jubilee line Westbound between Canary Wharf and Canada Water