From the North County to the South Bay, people reported hearing and feeling a big "boom" around 4:20 p.m. Monday.
San Diego firefighters received a number of calls, including one from their own Clairemont fire station, reporting the loud sound.
Users of NBCSanDiego sent in e-mails from Clairemont to Vista to Eastlake, reporting not only hearing what they described as a loud boom but also feeling it.
"It made the windows shudder," wrote Geoff Hurley of Eastlake. "It sounded like two booms with a few seconds in between each."
Cathy and Lee Smith of Simpson's Nursery wrote, "We just felt what could be described as a huge sonic boom or like an earthquake without the ground shaking in Jamul. Our house & windows shook enough to make me run outside!"
There were no reports of earthquake activity around that time according to the most recent data from the U.S. Geological Survey website.
Sheldon Miller in Bay Park described it as feeling like an earthquake also. "My house rattled and rolled," he wrote. "The windows shook and the house rolled, last 20 seconds but it was loud."
In the North County, one user compared it to military exercises he has felt in the past. "I heard that loud boom at about 4 p.m. all the way up here in Vista. It was louder than any training exercises I have ever known at Camp Pendleton and the ground and sky seem to vibrate some."
Also, the San Diego fire department contacted MCAS Miramar and Camp Pendleton to see if there was any training going on that might lead to that large of a sound. The spokesperson for San Diego Fire Rescue said they were told neither base has any activity happening currently that would cause such a boom.
The area has witnessed similiar so-called "mystery booms" before.
In April 2006, after a loud noise rattled the county, a local researcher pinpointed the source about 120 miles west of San Diego. The possible footprint of sound wave cutting across a military exercise venue known as Warning Area 291. But at the time, the U.S. Navy and the U.S. Marine Corps told KNSD there were no war games going on in the area at the time of the boom.
In August 1995, San Diego county residents felt and heard a strange rattle from downtown to Chula Vista and as far away as Palm Springs. Experts said then it wasn't an earthquake but was probably caused by a high-altitude plane.
The year before, in October 1994, people at the beach scrambled for cover when they heard a loud boom and felt the earth shake along Mission and Pacific Beaches. One store, "Gone Bananas" in Mission Beach, reported the glass from a window shattered into the street. The military wouldn't reveal if any tests were being conducted on local military bases at that time. North Island officials said the boom wasn't caused by their jets. MCAS Miramar officials said it wasn't their jets either.