Replying to Macbook Pro Randomly Shutting Off...

So i got this Macbook Pro the summer of 2008 before college. I'd say it is in 8/10 condition, never dropped it or anything, it's only been to class and with me in my room, etc..

The last week I have noticed that after a full charge (adapter light is green) and i unplug it to take with me somewhere, the battery symbol will say 2-3hrs remaining (depending if i'm watching/streaming something, just browsing, the usual). After maybe 15-30 minutes of use (batter bar still at LEAST half full (although it will say ~0:49 remaining) it will just shut off. The screen goes black and the only way to start her back up is by connecting it to a charger and pressing the power button, it will then reboot what i had left on the screen too.

It is not under warranty still, but do you think it's my charger? The only new apps i have on my comp  wouldn't do this.. (i did torrent two programs but i dont think thats it) but i have noticed some noise coming from the square box that is part of my charger unit, could that be it? just wondering if anyone has had or heard a similar issue before i take it to the Apple store...also this is preparing me to backup my comp, just in case (knock on wood). thanks.