Hey there,
You could maybe try Light portals over the openings of your windows in your interior scene.
They help force the computational power to focus on the room the light in the room and less on the whole scene. Remember, there is always noise in real life, so a little doesn't affect the image :).
Or take some out in photoshop with CameraRAW, Details tab/Noise reduction, at the sacrifice of a bit of detail.
Similarly use a little bit of denoise on the image in corona
Chris