YOU SAID:

I was walking to the door when I heard a noise.

INTO JAPANESE

音がしたとき、私はドアまで歩いていました。

BACK INTO ENGLISH

When I heard the noise, I was walking to the door.

INTO JAPANESE

騒音が聞こえたとき、私はドアまで歩いていました。

BACK INTO ENGLISH

When I heard the noise, I was walking to the door.