Deep, rolling sounds like thunder or a distant explosion, or signs that something might happen soon (like rumblings of discontent).
From Middle Dutch 'rommelen' or Low German, likely imitative of the sound itself. The word evolved from literal sounds to metaphorical meanings of hints or warnings.
A rumbling volcano gives literal rumblings, but when news reports mention 'rumblings of unrest in the city,' it uses the same word to describe early warning signs—humans borrowed how nature warns us to describe social movements.
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