automobile horn vs motor horn

automobile horn and motor horn both are nouns.

Word	Noun	Adjective	Verb	Adverb
automobile horn	Yes	No	No	No
motor horn	Yes	No	No	No

As nouns, automobile horn and motor horn are synonyms defined as:

automobile horn and motor horn: a device on an automobile for making a warning noise

Other synonyms of automobile horn include car horn, hooter, horn.

automobile horn (noun)	motor horn (noun)
a device on an automobile for making a warning noise	a device on an automobile for making a warning noise