The reader never knows for sure, but Scout's narrative leads us to believe that it is Bob Ewell prowling about outside Judge Taylor's back porch. Bob had been further disgraced at the trial, and he threatened to get even with Atticus as well as others: Following Tom's death, Ewell proclaimed that
... it made it one down and about two more to go.
It can be assumed that Atticus and Judge Taylor were the "two more to go," and it appears that Bob has murder plans for both of them. The scratching noise actually comes from the judge's dog, Ann Taylor, who is trying to get out of the screen door because she has heard noises outside. When Judge Taylor takes a look,
A shadow on the corner of his house caught his eye, and that was all he saw of his visitor.