Possible Clues:

• [I'm a pig!]
• Rooter's remark
• Grunt by a pig
• Noise sounds piggish
• Nothing in writing from 17 across, by the sound of it
• Sign of a pen problem?
• Cartoon sound of a pig
• What's produced by something found in pen, with ring on?
• Comment from the pen
• There's a beastly noise all right outside home