Perhaps she will just give one out for free when one reaches rank 3. However considering that as far as I know Murmur is the only weapon requiring a Runed Core it is probably unlikely.
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On top of needing to be rank 3 to buy the ruined core, I have heard that you can get a super rare drop from Crota that will allow you tu upgrade Murmur to an exotic at some point. I'm not saying this is fact, as its just something I heard during conversation when fighting Crota last night/this morning.
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Personally I am not sure I would want to upgrade murmur to an exotic form because of there already being a few primary exotics I like to use. We really need more epic special and heavy weapons.