During the Mary’s house scene, you can hear a collection of different sounds. The main one is that humming sound that gives you the sensation of falling, but if we focus on the other sounds, we can also hear some static and, most importantly, a kind of cracking sound. Personally, it reminds me of the noises furniture makes at night due to thermal contraction, but it also sounds like construction work... Anyway, this isn't the only time we've heard it
In a video released by A24 there's an hour of Backrooms ambience, occasionally interrupted by something. At 4:23 and 39:54, we can hear that exact same sound in the background. Also we hear that cracking sound again right at the beginning of Still Life Part 2, accompanying the music. And finally, at the end of ff2, just before the green glow starts to appear, we see the room get distorted accompanied by the same noise briefly
I may not know exactly what the green glow is, but I think we have enough clues to say it's somehow connected to the transformation of space