Yeah this is something I've noticed too and never really talked about because it sounds daft.
About three years back I was doing a session in an old farmhouse near Matlock and I started getting this low crackling static, almost like interference on a badly tuned radio, and it would build for maybe 10-15 seconds before anything came through. Happened four times that night, consistent as clockwork. At first I thought it was equipment fault but the static had a quality to it, hard to explain, like it had texture if that makes any sense.
My working theory, and its completely unprovable, is that whatever produces EVP is drawing on local electromagnetic energy and the static is basically the "charge up" phase. I've seen similar ideas floated around in relation to poltergeist activity too, where electrical disturbances precede physical phenomena.
Does the static on your recordings have any particular character to it? High pitched, low rumble, or more like white noise? Would love to compare notes because I think there might be a pattern here that nobody's properly documented yet.