Possible Clues:

• The horn of London, by the sound of it
• Honking excessively can start to grate
• Using one's horn
• Street noise
• What the motorist might be doing, by the sound of it, in 27 down
• Showing impatience as driver somewhere in London?
• Where to sound your horn in London
• The sound of the horn in London
• S. London district
• Having horns there in London