Nothing, there was a boy singing a Christmas carol.

From the text:

The owner of one scant young nose, gnawed and mumbled by the hungry cold as bones are gnawed by dogs, stooped down at Scrooge's keyhole to regale him with a Christmas carol; but, at the first sound of

'God bless you, merry gentleman,
May nothing you dismay!'

Scrooge seized the ruler with such energy of action that the singer fled in terror, leaving the keyhole to the fog, and even more congenial frost.