Was it a bomb? An exploded gas tank? A blown transformer? Neighbours in the 6000 block of Newlin Avenue in Uptown Whittier felt their homes and cars tremble and heard a deafening boom this weekend that sent them out into the street to investigate. They wondered aloud if it was kids playing with firecrackers, or something more sinister. Could it have been secret military operations? Aliens? The source remained a mystery Monday, according to police and fire officials. Neighbours said they heard the blast at about 8:45 p.m. Saturday but found no damage.