This is a common issue with #2 and #3 smokers and has to do with wood combustion. If your wood is dry, then you will experience the burps. Small burps are probably OK, but I have had some where it seems like the door is going to blow off. Some folks have advocated slowly ramping up the temp in the smoker; for example, set the temp to 150 and let it cycle through one before upping the temp to 225, etc. This gives the wood time to slowly warm up and begin smoking while avoiding the combustion.
The method that I have adopted and is working great is to put foil around the wood to diminish the air flow hitting the wood. I set the temp to 225 and let it heat up. I have not experienced the BIG burp with this method, although I will still get a couple "puffs" if the wood is dry. Give the foil method a try and I think you will be OK.