。
I heard a strange sound.
Literal
Strange sound [subject-が] was-audible.
聞こえる is spontaneous ('to be audible, to come to one's ears'), distinct from 聞く ('to listen actively'). With 聞こえる, the thing heard is marked with が as the grammatical subject, because the sound is what 'does' the appearing to hearing. English 'I heard' hides the subject; Japanese exposes it.