alarm bell

Short definition from Wiktionary

Noun

1. 1
   A sudden awareness of danger.
2. 2
   Used other than figuratively or idiomatically: see alarm, bell.
3. 3
   A bell rung to signal danger or emergency.
4. 4
   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.