This has been nagging at me for years actually. Back when I was doing some amateur EVP work out near Glastonbury, I kept getting these sharp bursts of static about 10-15 seconds before anything noteworthy showed up on the recording. Not during, before. Like something was warming up or pushing through.
I always assumed it was interference but the pattern was too consistent to ignore. Three separate sessions, same thing.
What recorder are you using? I've noticed it seems worse with cheaper digital recorders which some people say is actually a feature not a fault - the lower shielding makes them more sensitive to whatever field precedes these events.
Would be curious whether others are getting this in video too or just audio. I've got a mate in Bristol doing a lot of EVP work at the moment and I'll ask him to start logging it properly.