Yeah this happens constantly in my experience. The static isnt random noise either, its like the signal is being tuned, same as when you get interference on an old radio before a station locks in. I've always thought of it as whatever's communicating needing a moment to find the right frequency or something close to it.
Been doing EVP work for about 15 years now up in Lincolnshire and I can usually tell when somethings about to come through just from the character of the static. Theres a particular kind of layered hiss that basically tells me to pay attention.
Anyone tried comparing the static patterns across multiple recordings from the same location? I've got sessions from a couple of spots I revisit regularly and the pre-EVP static does seem consistent from visit to visit at each site. Would be interesting to know if others are seeing the same thing or if its totally different for everyone.