Original bug report:
For some reason I can't record sound of the game in my (potato) pc to show what exactly I mean. But! Here is "the sound" from another person's playthrough.
Explaining video's exact minute : Two dupes in the teleporter planetoid of Terra classic start. Both are sleeping. "Plop" sound (similar to what "finishing a building" sounds) but no building is currently built.
What I think is causing it: Plug slugs!
I think when the night cycle starts and plug slugs start searching (deciding?) for a spot to hang, they trigger the "plop" sound. It doesn't happen every time though (my pc manages to make most bugs buggier so maybe that is why I hear it often enough)
It's not a serious bug (nothing is crashing) just a sanity check (I'm not just hearing sounds... They exist!)
I generated multiple plug slugs and waited for night to fall. Usually there is a "plop"