fire alarm 
noun
A device making a loud noise that gives warning of a fire: a fire alarm went off and the building had to be evacuated...
- This time the ringing noise wasn't the fire alarm, but the dinner bell.
- That was when the idiots decided to test the fire alarm without the usual warning over the speakers.
- One girl bolted to the fire alarm and pulled it, setting the siren wailing and then sprinting for the door.