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"Susurro" Is not necessary something bad/disturbing, is just the fact of speaking something in very low voice, generally directly to the ear of the other person.
"Murmullo" In general, implies some kind of disturbing action. Usually not directly to the ear.
Hope it helps :)
Generally they're not interchangeable but there are some contexts where maybe can be used as synonims (I can't think of anyone right now). As you can see below susurro makes reference to a soft or quiet sound but murmullo has more the meaning of soft and continuous background noise. The difference is really subtle.
According to RAE:
For example you can say: El susurro de las hojas agitadas por el viento.
For example you can say: el murmullo del agua de un arroyo.