Some moan a lot. I honestly dont think they can burp. Because burping is almost the same reaction as barfing. They have to have the opposite reflex as the muscles use to swollow. And since they can not use their throat muscles to barf then how can they use them the burp.
You have to remember that they also have tons of acid in their stomachs which causes all the grumbling, you would think since they have all that acid they would be burping a lot, and in that case we wouldnt wonder. You know...
From east to west a travlin gypsy found her prancing pony for now their hearts run as one...into the north