Just wondering if anyone else has encountered this. I did a set of scheduled recordings where most (but not all) came back with a soft thumping sound covering 0 to 500 Hz. It repeated two or three times a second. Interestingly it would happen during the entirety of one scheduled recording and then not necessarily happen at all in the next recording. There were 5 second breaks between recordings, so it doesn't seem to be related to, say, a major hardware flaw.
Perhaps not coincidentally, this happened during the last hours of the battery power. This was the first time I'd let the batteries run out, so I'm wondering if there might be a correlation between the thumping sound and the low-power warning. I've always kept the LEDs turned off, so I didn't know I was toward the end of the batteries' life.