alarm-bell

noun3

1. •
   A bell rung to signal danger or emergency.
2. •
   [usually plural] (figurative) A sudden awareness of danger.

   • The specter of women in factories, women in schoolrooms, and women attending — a lot less addressing — public meetings set off alarm bells in the South. — 1998
3. •
   A bell which is rung to indicate a pre-arranged time, especially to mark a desired moment for waking up, performing a specific action etc.