alarm bell

from The Century Dictionary.

• noun A bell used in giving notice of danger, as from the approach of an enemy, from fire, etc.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Used other than as an idiom: see alarm,‎ bell.
• noun figuratively, usually in plural A sudden awareness of danger.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the sound of an alarm (usually a bell)