If it is - then it very well may be the reed valves opening up and dumping oxygen into the exhaust ports (which is what they're supposed to do). Dumping oxygen into the exhaust ports causes any unused gases to burn off, hence the popping (often refered to as backfiring).
If this is the case then it's nothing to worry about.
Some will say that you should remove the KLEEN system. However, I haven't found any good reason to do so.