What type of muffler are you running? Sounds like a Cherry bomb/glasspack. It's not unusual for those types of mufflers to occasionally pop, especially if the carb is not adjusted or jetted properly.
Another easy test:
Hold a dollar bill close to the exhaust tip while it's isling, as close as possible. Notice if it sucks the bill towards the pipe when it pops or at any other time while idling. If it does you've got an exhaust valve either sticking or isn't closing all the way.
Good luck
