That was it! When I unboxed the AudioMoth that I received yesterday, I immediately went to test it out at my desk...next to a wireless router. 😄 I do not plan to use these inside much, but is their a way to shield against electronic/radio interference?

Photos may help explain the battery pack bends and unevenness better... Is this as is expected?

Thanks for the clarification on NI-MH batteries. Yes, it is a new card. I reformatted it anyway, and still no improvement. I just sent you an email with a WAV file attached. I don't have much experience with building analog synthesizers, but from what I remember the last time I played around with building some, this sound I'm hearing in the AudioMoth sounds similar. That's why I'm leaning towards a faulty component or solder joint. And speaking of leaning, is the battery pack supposed to be unevenly connected to the PCB? Only one corner is touching (looks to be glued down). There is also a slight bend in the battery case. So much so that I can push down on one side of the batteries and the other will raise a centimeter or so (and then pushing down on the opposite side raises the other side).

I tried fresh Duracell 1.5V alkaline batteries but the sound did not improve. I'm still getting the same "feedback" as in the audio recording above. Furthermore, it appears that at least one other person is using NI-MH batteries successfully: https://www.openacousticdevices.info/support/device-support/recargeable-ni-mh-batteries

Batteries: Amazon Basics Ni-MH 1.2V 2400mAh

I'm using a SanDisk Extreme 32GB microSDHC UHS-3 Card with the following settings.