Very strange. Not sure what to make of that. Do you think it could just be a part of the wind gusts soundscape when you're outside? Do you think you could record it?

We played for a couple of hours and only heard it once, I don't think we'll be able to record it.

Quite the anomaly indeed. I haven't experienced this, so without further corroborating evidence or knowing what circumstances trigger it, I have to assume this is just a subtle element of the outdoor soundscape that is difficult to notice.