From Longman Dictionary of Contemporary Englishwarning bell/bellswarning bell/bellsused to say that something makes someone start to be worried or careful about somethinga warning bell rings/sounds As she read his letter, warning bells began to sound in her head. → warning
Examples from the Corpus
warning bell/bells• That telephone was sounding a warning bell.• When he sees the ride begin to fill with customers, Brown rings a warning bell.• If the engineer does not brake his train, a warning bell is set off, followed by an application of brakes.• If the defendant refuses to sign warning bells should ring.• At that moment no warning bells sounded in his head.• Broomhead heard warning bells starting to ring in his head.• Would she ring the warning bell that cushioned the little fellow from the worst of it?• Little wonder warning bells were sounding. 