Brewer's: Cracked Pipkins
Cracked pipkins are discovered by their sound. Ignorance is betrayed by speech.
They bid you talk—my honest song Bids you for ever hold your tongue; Silence with some is wisdom most profound - Cracked pipkins are discovered by the sound.
Peter Pindar: Lord B. and his Motions.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894