AUGUSTA, Maine — State police and other public safety officials say they have no explanation for a loud "boom" reported by residents of Kennebec and surrounding counties at about 9 p.m. Sunday. The sound, described by various people as similar to an earthquake, a plane breaking the sound barrier and an explosion, prompted dozens of calls to Kennebec County dispatchers and Maine State Police, according to Steve McCausland, spokesman for the Maine Department of Public Safety. McCausland said state police dispatchers checked with the Federal Aviation Administration about whether there were any planes down, and with the National Weather Service on an earthquake. They also monitored various fire frequencies, but as of Monday morning, he said, "There's no explanation as to what residents were calling about." Meteorologist Chris Kimble of the National Weather Service in Gray said Monday that he and his colleagues had only heard reports of the noise, "but we don't really know what it was or