Recently, having a weekend out in the mountains and scouting around with a microphone, I came across to this sound. From the distance it sounded like a quiet traffic noise, or distant train. When I moved closer to it, it came out that it was frogs mating in the pond.
All the pond was full of spawn. I regretted that I didn’t take hydrophone with me, it could sound interesting from that perspective.
Anyway, I will go back there later this year and see how it will sound then.