This typically happens when tempered fire glass is damp or wet and the burner is turned on. The heating of the moisture causes an expansion as it turns to a gas which may cause some of the cracked (but not completely broken) pieces of tempered glass to separate into pieces. This is a normal occurrence in new installations of tempered fire glass and is not a need for concern. Once all the cracks in the glass are separated into individual pieces the crackling and popping sound will cease. We do recommend that on new installations, if the glass is wet that you start off with a low flame and turn it up after the glass has heated and dried. To completely prevent this you can keep your fire glass dry by covering your fire pit when not in use (be sure it has completely cooled down to prevent a fire hazard).