Prolonged belching.

There’s a guy where I work that sometimes has these long rumbling belches that he seems not to notice. They don’t sound like ordinary belching, but are longer and quieter and continue even while he’s talking without having much effect on his speech.
I googled “prolonged belching” and found, among the usual gazillion hits, Aerophagia.
Some questions:
[li]What else could cause this symptom?[/li][li]Is the condition always self-inflicted?[/li][li]How common is it?[/li][li]Do you know someone who has the condition?[/li][li]If you do, what’s it like?[/li][/ul]
This fellow really doesn’t seem to notice.