One thing I have noticed with my ak100 is that if my phone is right next to it or very close to the unit and it receives a text there is crackling / distortion right before the text comes in. After the text displays on my cellphone, the crackling / distortion disappears. Before you send it away it might be worth checking to see if that is the issue.
I do notice the crackling sound once a while in some of my tracks. I thought it was a poor recording or due to sensitive IEM or both. Did worry a while if it was my unit. All the times that happen was when my AK100 is in my left pocket while hand phone was in my right pocket. But at that distance there shouldn't be any interference?