This evening I took the 1834 train from Reading to Guildford. Just as we were leaving Reading I could hear a loud tone which could best be described as the sort of noise BBC1 used to broadcast when it shut down for the night. I couldn't work out where it was coming from but I noticed that it seemed to stop when the doors were open. I have a recollection of previously hearing this before on a 165 or 166. Any idea what it might be?
I managed to record it on my phone. The background noise is the sound of the engines and the annoying hum starts at 21 seconds in just as the doors have closed
Annoying hum
