Trick is to take the car in when it is making the noise and leave it running until someone looks at it. Otherwise, it's nearly impossible to reproduce.
I cannot find a common thing I do every time i start the car when it starts crackling. It never happens mid drive, only directly after start and will continue until I restart the car. There is something magical between pressing start, accepting the idrive warning, watching the M video whenever it decides to play, putting the car into gear, and fiddling with the volume knobs. It isn't specific to one sound system or interior choice.
I'm almost certain it is linked to the engine active sound and the overall idrive system. Rev the engine and you'll get the crackle without touching any knobs. Don't rev and touch any knob and crackle city. I did notice that restarting without the idrive shutting off will still produce the crackle. Shut down, lock car (idrive goes off) restart and no crackle.
Next time it flips out I'll have to try to kill the idrive without powering off. Usually I can get it to power itself off if I plug in a certain USB stick with music on it. Not sure what that would prove but just more info to hopefully help get one more gremlin out...
