Right so this has been happening to me too and I only started doing EVP recordings about eight months ago so I'm very much still learning the ropes here. I picked it up after reading about the Pendle area having a strong history with all this sort of thing, seemed rude not to try it living here.
What I've noticed is there's this sort of crackle that builds up for maybe two or three seconds before anything intelligible comes through. I initially thought it was interference from my old digital recorder but I've since borrowed a different one from a mate and the same pattern shows up. That made me sit up a bit.
My understanding from what I've read is that some researchers think spirits need to draw on electromagnetic energy to actually manifest or communicate, and the static could be a byproduct of that process. Whether thats the case or its just the equipment I genuinely don't know. I'm curious whether people who've been doing this longer than me have any technical explanation for it, or is it just accepted as part of the process?