Yes, this keeps happening to me and I thought I was imagining it at first. I do most of my recording outdoors near the ley lines around Dorset and there's this very distinct crackle that comes through my digital recorder maybe 30-60 seconds before anything meaningful shows up on playback. It's not wind interference, I've ruled that out.
My theory - and I'm no expert so take this with a pinch of salt - is that whatever is trying to communicate is drawing on some kind of local electromagnetic energy before it can actually form into something coherent enough to register. Like a radio warming up almost.
Has anyone tried logging the exact timing between the static burst and the EVP capture? I've been meaning to do this properly but haven't been disciplined enough about it yet. Would be really interested to know if others are seeing a consistent gap or if its more random than that.