The clicking noise is your starter solenoid. It will only click once if your battery is outputting enough juice and then the starter will turn your flywheel, but if the starter is not getting enough juice, you get the repeated clicking. I guarantee you if you left your headlights on for a few minutes, the battery would have completely drained. A stock radio uses almost no current as well.
next time, make sure you turn your dome light off
That gets me the more often than everything else.