Moments after a mysterious sound was heard in San Diego, people started sending messages out via social media asking "what was it?"
NBCSanDiego received a number of reports from residents of a shaking or a loud noise just before 9 a.m.
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The U.S. Geological Survey has not reported a nearby earthquake.
Officials at MCAS Miramar said the noise or vibration was not a sonic boom from their aircraft.
Reports came in from different sections of the county including La Mesa, University City, Point Loma, Paradise Hills and La Jolla.
Marielle Bravo-Saltzman of Carmel Valley posted to NBCSanDiego's Facebook page, "I felt a shake at 8:38, but no boom."
Paul Prince of Pacific Beach sent a note to @nbcsandiego via Twitter "All my windows shook in the PB area..."
San Diego State University Professor Emeritus Pat Abbott felt and heard it and stepped outside his home.
“The only time you hear an earthquake is when you’re directly over the epicenter,” Abbott said.
In his opinion, the sound was atmospheric.
Thunder and lightning were in the forecast Friday, but NBC 7 Meterologist Jodi Kodesh looked into it and said there weren't any storm cells near the area producing thunder and lightning at that time.
When lightning strikes, the discharge of energy can be felt 5 to 10 miles away as a rumble or a boom, Kodesh said.
The closest rumble of thunder at the time would have been north of Los Angeles.
Sometimes, a military exercise known as chaff can cause a similar sound.
Strips of metal foil dropped by an aircraft are used to temporarily hide an aircraft from radar detection.
The National Weather Service couldn't confirm if the sound was a result of chaff and an MCAS Miramar official told us he was not aware of possible chaff in the area.