susurrate vs murmur

susurrate is not a noun while murmur is a noun.

Word	Noun	Adjective	Verb	Adverb
susurrate	No	No	Yes	No
murmur	Yes	No	Yes	No

As verbs, murmur is a hypernym of susurrate; that is, murmur is a word with a broader meaning than susurrate:

susurrate (noun)	murmur (noun)
	a complaint uttered in a low and indistinct tone
	a schwa that is incidental to the pronunciation of a consonant
	a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech
	an abnormal sound of the heart; sometimes a sign of abnormal function of the heart valves

susurrate (verb)	murmur (verb)
issue soft noises	speak softly or indistinctly
	make complaining remarks or noises under one's breath