Thanks for your suggestion, @tmikaeld. Here what I found, I tried disabling the wifi first and stressed the system, the sound still crackling here and there. Then I disabled the bluetooth and the issue was almost gone, it still happened 2 times during my 30 minutes test which means that not only the network IO causes the issue but also the whole system if under high CPU & Memory usage. It's a shame if it's a hardware issue. Apple should acknowledge this issue from us and find a solution.