Not uncommon... I couldn't do it either until I hit my early twenties. Instead of burping or puking, my throat would make this weird frog sound. It was uncontrollable, randomly sitting in class sounding like a dying frog.
Listen you defective basket case: burping is not an acquired skill. Infants burp. Toddlers burp. Lower orders of animal burp. It's a ready to go function inscribed on the lowest levels of The Human Firmware, and claiming that it took two decades to unravel the mysteries of an unconscious body function doesn't elevate you to the status of reliable source. I saw a picture of a slug pooping out of its face the other day and it could probably burp too.