Answer: the door knocker with his ears plugged
"It's very rude to stare" is what the knocker with his ears plugged says. All the door knocker with his mouth plugged can do is mumble. That is until Sarah removes the knocker to ask him a few questions. "Knock, and the door will open". Poor door knocker. Just after having his mouth free, the knocker is jammed back in his mouth, allowing Sarah and Ludo to go through.