Just a follow-up; looks like I spoke too soon. The crackling sound returned a week after I tried the "cleaning the SIM card fix". The strange thing is that it did work well for a while and now is still working but with occasional static. Again, I have an iPhone 4 so maybe this is different than the 4S issue but unfortunately it continues to be a problem.
Replace the metal SIM card bracket with plastic brackets and everything is OK. Why?
Because the pins of the SIM card touch the metal bracket, and therefore there is contact - a short circuit. Because it creates Static / Crackling Noise.
Today I've cut the SIM card into Micro, but I was still a little cut, so there is no contact with the metal bracket. Doing everything without problems. In the original micro SIM card (can see) contacts touch metal bracket and then creates a Static / Crackling Noise.
For me it's all right now. No More Static / Crackling noise and the sound is immaculate and clean.
Nice greetings from Croatian!