it sounds like a air valve which seeps constantly while the "pressure " in this "valve" does vary the sound does come and go a little bit.. disappear for a few seconds but always come back...
Hmmm . . . this sounds like the exact same problem I hear in "Nest". It starts up around the end of track 3, and continues through to the end of track 5. The entire album is bathed in forest environmental sounds, but these field recordings are quite noisy. There's something very raw about the recordings that I can appreciate though, and I figure that was Robert's intent, but the hiss during those parts is especially distracting.
Looking at the spectral analysis, those tracks have a wide band of noise covering 3.5-10k range. From your description, this seems like the hissing sound you're talking about. As you said, it's not a consistent hiss. It sort of reminds me of bad noise reduction where the noise is sitting on the threshold of being removed or not.
The noise actually extends beyond that to the ceiling, but there's a clear drop in gain beyond the 10k range. It looks similar to MP3 compression, but without the smearing.
I wonder if it's a bizarre chorus of bugs of some kind.