In my experience they have been distinct sounds of pressure and movement between plant fibres, in ways that the wind or insects wouldn’t or couldn’t cause in that setting.
If you lay still on a warm, quiet, still day in spring in a field or meadow, you might hear a lot of sounds you otherwise haven’t and wouldn’t expect. There’s a lot of activity. Sometimes plants will let some leaves or blades free after a seed husk finally breaks under their weight. It’s just gravity and pressure at work, but you’ll hear these things frequently.
This sort of thing reminds me of learning to find mushrooms. I had no idea mushrooms are everywhere until I started looking. They occupy all kinds of spaces in various seasons, dependent on a rain, a species of tree, or a sudden warmth after a cool period. Once you see them, they’re everywhere.
Similarly, plants and their myriad processes kind of jump out at you once you observe them closely. Before then they may seem relatively inert.
