Maybe? I'm not sure. I haven't played campaign yet; he could be playing an older version.They removed this sound effect recently (thankfully). So it shouldn't be that on a current version of the game. (Might come back later, who knows.)
Maybe ambient engine noise....?
The "Unstable" is currently the same as the main branch (0.8.2). I noticed it had stopped in 0.8.0.Well, in the current unstable release anyways. I just noticed it yesterday. And in my game it was accompanied by a blue flash of light (like lightening) off in the distance. I went looking for it, but never found out what it was.
Maybe he was near the pillars biome,with those...plants.