A loud, deep ringing sound like that made by a bell or metal object being struck.
Likely imitative in origin, from Middle English, related to 'clang' and 'clong' as onomatopoeia for metallic sounds.
Clong is one of those beautifully onomatopoetic words where the sound itself tells you what it means—if you say it aloud, you can almost hear the resonance of a struck gong!
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