My 2002 Kia Rio is making funny noises. What could it be?

For about the past year, my car has been making funny noises. I would describe it as a low groaning noise. The groaning noise is very loud from inside the car, so loud that I have to turn the radio up really loud in order to hear the music. It cannot be heard from -outside- the car, however. Recently, a new sound has added to the mix, sort of a rhythmic rumble. This seems to be coming from underneath the car somewhere, and in the back, not underneath the hood. The check engine light comes on occasionally, but then it goes off again. My car is a 2002 Kia Rio. I just put a new battery in it today, got a recent oil change, and had some repairs done on it last year due to overheating problems because the fan broke. The car itself seems to run just fine, and I have not had any problems, except for this noise. What could be causing these strange noises, and what should I do about them?