Loud explosion in North Hollywood still a mystery, police say

So what was it?
The loud explosion that jolted North Hollywood on Tuesday night remained a mystery the next morning, authorities said.
An officer at the Los Angeles Police Department's North Hollywood station said police searched the area for a half an hour Tuesday night but had no luck in determining what caused the sound. The officer said police received numerous calls about the noise.
At about 9:30 p.m., numerous people began tweeting about a loud explosion. Some thought it had come from a North Hollywood Metro station; others reported hearing an explosion in Studio City.

Source:  http://latimesblogs.latimes.com