Yeah this is something I've noticed a fair bit actually. Running sessions up here in Northumberland at a couple of old sites and before you get any clear EVP there's almost always this short burst of static, lasts maybe 2-3 seconds, then it drops off and thats when you sometimes catch something on the recording.
My working theory is it's related to EM interference - like whatever's happening to produce the EVP is also affecting the recording equipment beforehand. Sort of a precursor signal. Some of the abduction literature talks about similar EM spikes before contact events which I find interesting, makes me wonder if its the same underlying mechanism just in different contexts.
Anyone else logging the timestamps when the static hits vs when the actual EVP appears? Would be worth comparing data across different locations to see if the gap is consistent.