Probably just something you ate... of course, if you did just eat, may not want to look at this potential diagnosis. Spoiler
First of all, how long has it been like this? Did you do or eat anything that's out of your ordinary choice of foods in the past day or so? What about stress? It is almost impossible for anybody to tell you what's going on if you ask on a msg board. I would first get some anti-acids, especially ones that have simethacone [main ingredient of Gas-X]. Take that for a day or so and see how you feel. If this problem is consistent, you may want to go see a physician.
"I don't know why my stomach be makin' that noise!" "'Cuz you hungry, b*tch! " <object width="420" height="315"><param name="movie" value="http://www.youtube.com/v/-k6wRb8SkIg?version=3&hl=en_US&start=65"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/-k6wRb8SkIg?version=3&hl=en_US&start=65" type="application/x-shockwave-flash" width="420" height="315" allowscriptaccess="always" allowfullscreen="true"></embed></object>