Hi. What you are experiencing is normally due to a partially clogged drain. When you flush the toilet, the water hits the clog and bounces back with an air bubble in front of it. That is the burp you hear. The solution is to first try plunging it really well with a good plunger and flushing. Repeat a couple of times. If this doesn't help you will need to run a snake down the drain.
You can buy what is called a "closet auger" for around $30 at your local home improvement store. This is used to snake the toilet. And since a plumber will charge likely twice that just to show up at your house, I'd try that if the plunging doesn't work. I hope this helps you in saving some money and solving your problem. Take care.