I've heard this same digital noise here in Hot Springs Ar too and always wanted to know what it was. On mine a PL tone pops up with the digital noise transmission.
That same noise comes up on a vhf freq here in central ar and i have never heard any voice traffic yet.
When the noise starts its motorboating a PL 67.0 appears. I'm not sure if its encryption or a system thats not monitorable. May even be some sort of a data being transmitted.