gm/pontiac tech/expert help with BCM

okay guys I've got a 05' sunfire, I am trying to diagnose. Car will start up with a jump, and then keep running, tested alternator, and reads 14.8v, so no problem with charging system. when I turn it off it won't start back up and when I test battery voltage you can see it going down quick on my voltmeter, I also can hear a clicking sound that I believe is coming from the bcm even without the key in the ignition, I am unfamiliar with the type of system I am a UTI student and have basic knowledge of circuits, relays, and such. Any help is greatly appreciated. also, I don't have access to a scan tool, but the MIL is on... been doing some research and believe it could be something to do with the passlock system, and from what i hear theses systems are a SOB. anyone seen similar issues