Always wondered but never got around to asking until now....does anyone know what that low-frequency audible "hum" sound is after every transmission on the WCCCA system? (I think the analog 800Mhz Portland system used to do it to?) If someone with technical knowledge of how these analog trunking systems work happens to know the technical answer, I'm quite curious. You can hear it in this recording from the attempted helicopter hijacking at KHIO a few years ago.