Several residents in San Diego County reported witnessing a strong rumble that shook their homes Monday morning. NBC 7 first heard calls from locals wondering what it could be just after 9 a.m. Other San Diego County residents have written on the NBC 7 Facebook page that they also noticed the strange occurrence. "My front door started rattling with no discernible wind," commented Pilar Shepard. "I thought maybe we'd had a small earthquake." The U.S. Geological Survey, the agency that monitors earthquake activity, has no reports of a quake in our area. Facebook user Tania Goddard said she felt the rumble in El Cajon."My sliding window and front door were shaking," Goddard commented Monday. "There was absolutely no wind outside." Facebook user Nina Gardner said she heard it in Ramona. "I heard about 6 booms," said Gardner. "They were very loud and shook my windows. Happened about 8:30 a.m." Reports like this one aren't uncommon, and military aircraft with supersonic capabilities frequent