Silence, by definition, means a complete lack of noise.
Hush is an onomatopoeic utterance (such as moo, ping etc), a verb, calling for silence:
"Hush, I can't hear what my friend is saying."
It has, however, become used as a noun occasionally - not in the sense of absolute silence but as a much lower level of sound:
"A hush descended upon the audience as the lights faded."
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