I have this problem too! Although I have noticed it's lessened a little bit since I've been on the IBS diet. I don't know if it's caused by anything I'm eating or just a nervous habit...
Could it be you're just swallowing a lot of air? I know I do that when I'm anxious or just drinking from a glass/cup. A friend recommended breathing out before I take drink and that actually has helped!
I don't know that you use any of these, but anti-diarrheals like Immodium can cause burping and bloating. I've also heard that some other meds and supplements, like pain killers, fiber and iron, can cause an increase in gas which leads to burping.
Hope this helps. At least they're silent and don't hurt, right?