It’s the onboard mic. There is also some freezing of the screen (although very short; about less than a second) at the beginning of the tape.
There was this time that the footage has blue horizontal stripes (the same blue color when a camera is in VTR mode), as if the “scanning mechanism” in the alternate field (or whatever it’s called) is not working. It happened just once and never again occurred. Although I get a bit worried because it might happen again when shooting a very important event.
I do not know if all these are related to the popping sound, like if they happen because of the tape head or recording mechanism.