I've had this too.. but also gurgling noises, even when upright. I shouldn't be proud of this, but I can burp like I breath out of my lungs...
It started from taking too many pain killers.. or I should say, enough pain killers over a long period of time (they were for an unrelated ailment). They caused my stomach to produce more acid, which in turn attacked the sphincter it my throat. The acid then flowed up and damaged much of the tubing. Small lumps formed which obstructed the flow of food making it hard to swallow, and making me feel like I was gaging near constantly.
Simple solution.. don't eat so much meat, lay off any drugs (like pain killers), and eat watermelon, lemons, grapefruit (pretty much any normally acidic foods - they end up causing your body to produce a base to digest them).
After few days like this, my problems started fading, and only a few weeks later, they were gone. Hope everyone else has similar luck.
So, now I get the benefit of easy burping, without the trouble of acid reflux or esophagus pharengeal (or however you spell it) problems.