Originally Posted by The Dude
It's all stock
Hmm. You have a 21-22 year old stereo in your car then. It could be a lot of things. My first thought is the fact everything is 21-22 years old is probably the first issue you're having. Personally, I wouldn't even know where to start looking. Is the crackling/popping coming from any specific speaker the most or is it in all of them?
If it is occurring in all of them I would imagine there is a problem with the head unit/receiver itself or there is a short in the amplifier (at least I believe there is an amplifier in the e28 for the stereo).
If the crackling is audible in a specific speaker then there is the possibility that speaker is on it's way out.
One last question though. Does the crackling/popping persist when the stereo is on or does it only occur when it turns on?