
Ok, here's my reason for deciding on the title 'Heart silent'. Besides it being the title used in foxtoon, the actual title in Korean isn't exactly coherent either.
If the author had intended it to be a full sentence, it would have probably been written as : '심장은 조용하다' (the heart is silent).
As others have already mentioned, the literal translation would be heart silently, in which the author chose to write it that way on purpose, probably cause it sounded better as a title in Korean, or left vague intentionally, for example: ' the heart is silently...'
Thus heart silent would make more sense in this case in English.
W_W I know this phrase is hard to translate from Korean...but can't they change the name? It could be something close like "Be Still My Heart" instead of this ridiculous nonsense.