Alright, I'll start off by announcing that I'm new to car maintenance in general, a 20 year old newbie. That being said, here's the problem. I bought this car a few weeks ago from a private party. Its treated me great until the check engine light came on a few days ago just as the odometer hit 125000. Last night when I came to start my car, it made a rapid clicking noise, that would suggest the battery was dead. I didn't leave my lights on or anything, and it had been driving fine just before that. So I had a friend give me a jump and it started and I drove it home and it drove without any problems. (I should add that the check engine light went away at this point). Now this morning I tried to start it again and it had the same problem. I noticed that the lights and radio work just fine. Additionally I should add that I had my itrip fm transmitter plugged into the cigarette lighter while the car was off, but the ipod wasn't plugged in.

What do you think it could be thats causing this problem? Thanks in advance.