I have this same problem and have been doing it for over a year and a half now. I probably burp 100s of times a day - but never loud. Just little quiet burps that rumble my stomach and make me feel mildly nauseous. I don't get heartburn and it doesn't matter the time of day. I've searched extensively about this online and come up with only one thing that makes sense for my situation. I think I have post-nasal drip that is causing my burping. Post-nasal drip (PND) occurs when excessive mucus is produced by the sinuses. The excess mucus accumulates in the throat or back of the nose. The mucus then runs down the back of your throat and into your stomach - taking air with it. This excess air is causing the burping.
It may be a stretch but its the only thing that makes any sense for me. I do clear my throat a lot, swallow a lot and constantly have the feeling of "junk in my throat". Before allergy season gets bad I'm going to go to the doctor and get allergy medication to see if it helps the burping and the mucus problem.
Get a test done called a Heidelberg Gastrogram (google this) Good luck!