YOU SAID:

i was walking down the street when i heard a loud noise

INTO JAPANESE

大きな音を聞いたとき、私は通りを歩いていた

BACK INTO ENGLISH

I was walking down the street when I heard a loud noise

INTO JAPANESE

大きな音が聞こえたとき、私は通りを歩いていました

BACK INTO ENGLISH

I was walking down the street when I heard a loud noise