Why It Happens
As water is added to the black tank, air must be removed. Since it's an enclosed tank, there is a vent that will allow that air to escape. A burping toilet is a sign that the vent is not able to expel the air for some reason. If you're noticing that it happens when your tank is near full, it's most likely that the vent's pipe sticks too far down into the black tank. When water covers the opening of this pipe, it can no longer allow air to escape. The only other possible place for the air to go then is back up into the toilet. This bubble can not only splash things back out of the toilet, but means that odors from the tank are being put into your bathroom as well. If you are finding that you are getting a burp all the time, no matter how full your tank is, then you most likely have a clog in this vent somewhere. This can be caused by letting the tank get too full, and then things like toilet paper get stuck in it. This vent leads all the way up to the roof so to remedy the problem, you'll have to hop on up there!
How to Fix it
If the burping only happens when the tank is near full, then you want to start emptying the tank sooner. Because the pipe is down in the water, you're risking a clog which will then cause more problems that require more work. Flush your tank with clean water each time you empty it to make sure you don't end up with a build up of dried things on the bottom of the tank (also referred to as piling by some). That build up means less room in your tank and more frequent dumps. It also means an increased possibility that the built-up sediments can become dislodged and cause more clogs. If the burping is happening even when the tank is empty, it's time to tackle the vent and get it cleaned out! You will need a flashlight and a garden hose hooked up to a water source. Once you're on the roof, take a look down into the vent with the flashlight to see if there is anything in there. You never know if there is a bird's nest or other debris in there. Using a garden hose, flush out the vent from the roof. You may need to repeat this a few times to get the entire clog. If the water backs up in the vent as you add it in there instead of making its way to the black tank, you have a pretty bad clog that may need more muscle than what the water provides. A pipe snake will come in handy as you can snake it down the vent to push out whatever may be stuck. Then flush the pipe again.