What is this weird animal sound I recorded?

As Carla and I walked through a banana grove on Madeira island in early March, we heard an unusual animal sound. It was coming out of an irrigation pipe, about 3 or 4 inches in diameter. Whatever it was, it was far back in the tube, so I couldn't see it. I recorded about 30 seconds of the sound it was making, which you can listen to here. It might be two or more animals arguing. If you know what kind of animal it is, please tell me in the comments.

[This post originally appeared in a recent issue of my newsletter, The Magnet. Subscribe here.]