Hey Alex, Its in most of the recordings but I think you're right that it might be the environmental sound. I just made a recording of my voice and I no longer hear the static noise. Its just that I was on site in the first recording and dont remember hearing much background noise but there might have been an insect near the AudioMoth that i didnt take note of. The SD card im using is the SanDisk Extreme 32 GB micro SD and for the batteries, some of my AudioMoths were running on Duracell AA batteries and some on the rechargeable Fujitsu AA batteries. Cheers, Ben