murmur vs susurrate

murmur is a noun but susurrate is not a noun.

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

As verbs, susurrate is a hyponym of murmur; that is, susurrate is a word with a more specific, narrower meaning than murmur:

Other hyponyms of murmur include coo.

murmur (noun)	susurrate (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

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