Blown speakers
Don't know if you have a Z3 or 4. I have a 4 and one of the speakers directly behind the driver's seat started to "rattle". I took it to
Critz and they said it was blown. Fortunately, I was under warranty and it was no cost. But, I had to leave the car for most of the day, so it maybe took two minutes or two hours. I suspectit was pretty easy to get out since otherwise would have meant removing the entire cover behind both seats and I saw no signs of that.
So, my "guess" is that it either pops or twists out. In any event, you will need new speakers.
You might go to Barnes/Noble and look for one of the auto repair manuals for BMW (don't buy it, just scan) and that may have some directions as to how removal is done. Just a thought.