I guess this is LT1 tech since its about my Caprice which is a mini-LT1.

I forgot to plug in the sensor under the MAF, I started the car and the Check Engine light came on. I plugged it in, but now there are rattling noises coming from under the hood when I turn it off. I dunno if I just never noticeed the noise before, but is the light on just cause I forgot to plug the sensor in?

 

That is most likely what set it off. Try disconnecting your battiers negative terminal, and let it sit for 20 mins. Reconnect and drive around for a while. If no light occurs, then that was the problem.