http://www.nbcsandiego.com/news/local/Strange-Sound-Reported-San-Diego-Coastline-Mystery-Sound-160873565.html?dr

The mysterious sensation was described by some people as sounding like a door slamming while others said it was strong enough to rattle windows.

A check of the U.S. Geological Survey website showed no earthquake activity.

NBC 7 San Diego's Dagmar Midcap was in Del Mar at the time and described it as a "Sonic 'rumble'" She tweeted, "according to my contacts at USGS, not seismic but rather sonic."