Harry has always been very sound sensitive and very inquisitive, so every strange noise always has to be fully investigated.
Sometimes his curiosity can get him into difficult situations. This time he was so desperate to find out where the strange quacking noise was coming from that he insisted on trying to climb over the wall to see what it was.
The duck got such a fright when he saw Harry hanging down the wall staring at him, that he immediately stopped quacking.
Harry hanging over a wall