。
She comes to know the faint flutter of first love.
Literal
She [topic-は] faint feelings-of-love [object-を] know [come-to-ようになる].
ほのか ('faint, dim, subtle') is a な-adjective with a poetic, soft quality — used as much for sensory faintness (a faint scent, a dim light) as for tender feelings. 恋心 ('feeling of love, romantic affection,' literally 'love-heart') is a literary noun, more delicate than 恋 alone. ~ようになる describes a gradual transition into a new state or ability — 'come to do.' The plain non-past 知るようになる reads as either a future development or, in narrative context, a generalizing description of the character's emotional arc.