Yeah this happens to me constantly and I've never been able to work out if it's a genuine precursor to something or just the recorder picking up interference right before I speak or move.
Did a session at an old mill near Clitheroe last autumn and caught a really clear class A EVP, and going back over the recording there was definitely about 3-4 seconds of this weird crackling static right before it. Wasn't wind, wasn't clothing rustle, I know what those sound like by now.
What recorder are you using? I've had it on both my Zoom H1 and an old Olympus digital voice recorder so I dont think its equipment specific. Makes me wonder if theres some kind of energy fluctuation happening that the mic is picking up before whatever causes the EVP actually manifests properly. Would love to know if anyone's managed to capture both the static burst and the EVP on separate recorders running at the same time, that would basically confirm its not just a recording artefact wouldn't it.