I've heard of this crackling/popping in HK cars on other platforms and my F87 has it too and I believe it is the amp. Here's one from a quick search (
https://g20.bimmerpost.com/forums/sh....php?t=1781851) which also says it's an amp. Trouble is that's not an easy thing to swap out and try, you'd have to get a dealer to be willing to try it.