warning signal vs alarum

warning signal and alarum both are nouns.

Word	Noun	Adjective	Verb	Adverb
warning signal	Yes	No	No	No
alarum	Yes	No	No	No

As nouns, warning signal and alarum are synonyms defined as:

warning signal and alarum: an automatic signal (usually a sound) warning of danger

Other synonyms of warning signal include alarm, alert.

warning signal (noun)	alarum (noun)
an automatic signal (usually a sound) warning of danger	an automatic signal (usually a sound) warning of danger