Piccadilly Line to Heathrow Terminal 3, Zone 2, 18:30pm

I suddenly realise what the sounds of the London tube are. The reason I finally learn perhaps is because I am listening to them when the tube is running in an open space. The sounds are: 1) the high pitch of wind (because the train runs); 2) the low pitch of train running on the rail track; 3) The announcement broadcast by the tube driver; 4) the conversations and sounds made by the passengers.